slug invader

I just found a slug on top of my main aquarium. Crawling across the hinge of the lid right where it meets the edge of the tank. A SLUG. Was it trying to get in? How did it get all the way up there?! I looked and can't see a slime trail anywhere.

This guy. For a sense of scale, that's the texture of a napkin it's sitting on. A small slug. What the heck was it doing. I don't even know how it got in my house, much less climbed up on top of the tank! My best guess is it was hiding under a pot that I brought inside last fall, and finally decided to look for better living space...

I swear it was looking at me in when I took its picture.

So here's a question that never ever crossed my mind before- would slug slime harm anything if it got in the tank? Ugh.