What do you call the punctuation mark on french word crudite?

The punctuation mark on the French word "crudités" is called a diaeresis.

It is a pair of dots placed over a vowel to indicate that the vowel is pronounced separately from the following consonant.

In the case of "crudités," the diaeresis over the "e" in "crudités" indicates that the "e" is pronounced as a separate sound, rather than being silent or part of a diphthong.