This is a slug. Photo is blurry.

Hi Temcol, it's mucus (slime) that the slug is hanging from. According to a wildlife programme I once watched, it's to do with mating (if I've remembered correctly)!

I once took a photo of one abseiling down into my greenhouse from the roof vent! I couldn't believe my eyes! The blooming thing had climbed all the way up the glass on to the roof and was letting itself in like a burglar!

Welcome to GoY.
as said it is mucus. depending on the species this could be its mating strategy. Or just its way of getting around from high to low places.