the fuzz stuff that get's on gravel from uneaten fish flakes..will mystery snails eat that?

I don't really know what you mean by fuzz stuff.. but it sounds gross.

Brigs (Pomacea bridgesii, sold as "mystery snail") will eat dead or rotting matter. Of course, if it's just some disgusting stuff.. I don't know.

Do you mean more like algae or is it the fish flakes that have absorbed water and fluffed?

fish flakes that drop to the bottom,and become fuzzy looking.

Well i think you should just clean this up, if it stays to long in the
tank it's making your water dirty... don't forget the 5 min rule
what ever the fish don't eat take it out.

nothing to clean up. i was just asking if they would eat it.

Snails may eat it but I dont know how good it will be for them. I love my snails as much as my GF, so what I wouldnt feed to my GF I would definately not feed to my snails.

In my experience, snails will eat fallen food before it gets to the fuzzy stage anyway