Learn French: Cette étoile de mer est tout humide, mais très jolie. - This starfish is completely damp, but very pretty.

This is the tricky rule for the adverb tout (meaning completely or very). It only adds an 'e' for feminine adjectives if they start with a consonant or an aspirated 'h'. Because humide starts with a mute 'h' (which acts like a vowel in French), tout stays in its default, invariable form.

Do I pronounce the 't' at the end of tout here?

Yes, you do! Because humide starts with a mute 'h', the word acts as if it starts with a vowel. This means you must make a liaison, and you will hear the 't' connect smoothly to the next word.

How do we know the noun étoile de mer is feminine?

The word étoile (star) is feminine, which we can clearly see from the feminine demonstrative adjective Cette (this). The phrase de mer (of the sea) just tells us what kind of star it is, but the main noun controls the overall gender.

Why does jolie have an 'e' at the end?

It is an adjective describing the feminine noun étoile de mer. Even though it comes later in the sentence after the word mais (but), it still needs to agree in gender with the starfish.