It looks like some snail poo got stuck. Pick the snail up and take a paper towel and wipe it off. It should come off. Snail poo usually comes out in strings of little whitish, elongated lumps. Look almost like eggs in a string but smaller. This is going to sound gross but look at the stuff and kinda crush it in the paper towel. If it falls apart and doesn't look like there is any slime inside of it, than it is just poo. I suggested the last part because parasite eggs are always a possibility in any animal. Eggs will be soft, slimy, and/or gelatinous.