And grubs have their place, they're gross because of the hard head attached to the soft body. But that would work better if players had to roll over them or fall on them. For something creepy to crawl up you I can't think of anything better than a slug.
So how about same details but it's a slug:
The dun slug has but one aim, to lay eggs in the mouth of a living creature. It is the size of a cat and can crawl up walls and swim slowly. Attempts to harm it in any way inflict the damage on the wielder instead.P.S. go to Die Nacktschneckenfauna des Russischen Reiches (1901) for all your color slug public domain needs.