I woke up at 3:00 a.m. this morning and since I couldn't fall back asleep I decided to get up and start my day. When I flipped on the lights in my office, I glanced at the tank and saw what I thought was a part of the anemone's foot stuck to the glass beside it. As I watched, the foot thing suddenly took off. It moved down the side of the tank, slid across the bottom, and went to go hide under a rock. I've never seen this thing in my tank before and I have no clue what it is. The thing is about the size of a half dollar, white and completely flat in appearance and moves like a slug/snail but really fast! Any ideas?