WHAT, a slug??!

Before getting my first cup of coffee at 6AM, I switch on the grow lights, feed the fish, and examine the grow bed. At first I thought, that's a thick gray ​root climbing over the hydroton. A bit too thick, as it's growing near the new arugula sprouts. Hmm

​Then, it looked at me.. WHAT?

Oh dear, it's a slug??!! Where did it come from? This is an indoors aquaponics system.​

​Strange looking body, with a strange hole on the side

I used a little stick to pick it up, and killed it with a single spray of window cleaner.. It died faster than I expected.