Learn German: Du kannst dem Geruch ruhig vertrauen, der Pilz ist gut. - You can safely trust the smell, the mushroom is good.

As an adjective, ruhig means "quiet" or "calm". But here, it acts as a modal particle. It adds a comforting tone of reassurance, giving the listener permission or encouragement to do something without worrying. In this context, it translates closely to "go ahead and" or "safely" (as in "you can safely trust").

Why is it dem Geruch instead of den Geruch?

The verb vertrauen (to trust) is a dative verb, meaning it requires its object to be in the dative case. Since Geruch (smell) is a masculine noun (der Geruch), the definite article changes to dem.

Why are these two phrases just joined by a comma? Shouldn't there be a weil or denn?

In spoken and casual written German, it is very common to connect two main clauses with just a comma when the second clause clearly explains the first. You could absolutely say ..., weil der Pilz gut ist, but leaving the conjunction out and just saying ..., der Pilz ist gut sounds very natural and conversational.

Is gut normally used to say a mushroom is safe to eat?

Yes! When foraging for mushrooms, calling a mushroom gut simply means it is edible and not poisonous. You could also use the specific word essbar (edible), but gut is universally understood in this context.