Learn Russian: Помимо кислоты, наш крем отлично увлажняет лицо. - Besides acid, our cream perfectly moisturizes the face.

This is driven by the preposition помимо (besides or apart from), which is the focus of your current lesson. It always requires the Genitive case. Since кислота (acid) is a feminine noun ending in -а, it changes to -ы in the Genitive.

Is the comma after кислоты strictly necessary, or is it just an optional pause?

It is mandatory! In Russian punctuation, prepositional phrases that express addition, exception, or inclusion—like those starting with помимо, кроме, or включая—must be set off by commas when they appear at the beginning or middle of a sentence.

Could we use наряду с instead of помимо in this sentence?

It would sound a bit unnatural here. While both mean 'besides' or 'along with', наряду с (which takes the Instrumental case) is used when two things are given equal weight or status, like two different groups of people participating in an event. Помимо is much better for simply listing an extra feature or ingredient, essentially meaning 'in addition to having acid, it also does this.'

What is the root of the verb увлажняет, and how can I remember it?