Мало того что фикус начал вянуть, так ещё и один его лист стал жёлтым.

It is a fixed Russian pattern meaning something like:

• not only ..., but also ...
• as if it weren’t enough that ..., ... as well
• on top of that ...

So here the speaker is piling up bad news:

• the ficus started to wilt,
• and on top of that, one of its leaves turned yellow.

Literally, мало того is something like that’s not all / that’s not the end of it, but in real usage you should learn the whole pattern together.

Because мало того что is often treated as a single conjunction-like expression.

So the sentence is structured like this:

• Мало того что фикус начал вянуть,
• так ещё и один его лист стал жёлтым.

In other words, the main break is between the two clauses, so the comma goes before так ещё и.

You may sometimes see Мало того, что ..., with a comma after того, if the speaker/writer wants a stronger pause there. But in your sentence, мало того что is being used as one unit.

Because this sentence has two clauses:

1. фикус начал вянуть
2. один его лист стал жёлтым

The expression мало того что ..., так ещё и ... links those two clauses, and the comma separates them.

So the pattern is:

Мало того что X, так ещё и Y.

After начать / начать meaning to begin/start, Russian normally uses an imperfective infinitive.

• начал вянуть = started wilting / began to wilt

That is why вянуть is used, not a perfective verb. The idea is the beginning of a process.

This is very common in Russian:

• начал читать = started reading
• начала плакать = started crying
• начало темнеть = it started getting dark

Вянуть means to wilt, to wither, to droop.

It is especially used for:

• plants
• flowers
• leaves

So for a ficus, начал вянуть is very natural: the plant is no longer healthy and is starting to droop or lose freshness.

It is more specific than just умирать (to die). A plant can вянуть before it fully dies.

Because after стать meaning to become, Russian normally uses the instrumental case for the predicate noun or adjective.

So:

• стал жёлтым = became yellow / turned yellow

Here:

• жёлтым is instrumental singular masculine
• it agrees with лист

This is a very common pattern:

• стал большим = became big
• стала красивой = became beautiful
• стало трудно = it became difficult

So лист стал жёлтым is the standard grammar.

Yes, absolutely.

Both are natural, but they feel slightly different:

• лист стал жёлтым = the leaf became yellow / turned yellow
• лист пожелтел = the leaf yellowed / turned yellow

The version with пожелтел is more compact and uses a single verb.
The version with стал жёлтым describes the change through becoming + adjective.

In many contexts they are almost interchangeable.

Yes, that is the idea.

• один его лист literally = one leaf of it / one of its leaves

Here его means its because it refers to фикус.

A slightly more explicit way to say one of its leaves would be:

• один из его листьев

That version emphasizes more clearly that the plant has several leaves and one among them changed color.

But один его лист is also natural and understandable.

In Russian, его can mean both:

• his
• its for masculine or neuter nouns

Since фикус is a masculine noun, его here means its.

Examples:

• его книга = his book
• его окно = its window, if referring to a neuter noun like здание
• его лист = its leaf, if referring to фикус

Russian does not have a separate word exactly like English its.

Фикус is a masculine noun in Russian.

That affects verb agreement in the past tense:

• фикус начал вянуть

The past-tense verb начал is masculine singular to match фикус.

If the subject were feminine, you would get начала:

• роза начала вянуть = the rose started to wilt

So this is just normal past-tense agreement.

Так ещё и adds emphasis. It makes the second problem sound like an extra annoyance or an additional bad development.

Compare:

• ещё = also / still / more
• так ещё и = and on top of that / to make matters worse / what’s more

So in your sentence, it is not just saying also in a neutral way. It sounds more like:

• the ficus started to wilt,
• and to make matters worse, one leaf turned yellow.

The little и at the end strengthens the feeling of addition.

The overall pattern is fairly stable:

Мало того что X, так ещё и Y.

So you usually keep that frame. But inside each clause, Russian word order is flexible.

For example, the second clause could be rearranged a little for emphasis:

• так ещё и один его лист стал жёлтым
• так ещё и жёлтым стал один его лист — more marked, more literary/emphatic

The version you have is the most neutral and natural.

It is mostly neutral written/spoken Russian with a mildly expressive tone.

The construction мало того что ..., так ещё и ... is common when someone is complaining, emphasizing a chain of problems, or dramatizing a situation a little.

So it is not slang, and it is not especially formal either. It is everyday educated Russian.