They slime up a bit don't they? i've only handled a carpet once when I worked at an LFS and since then realizing I was sensative to its sting asked other employees to do so afterward. its possible its just released something gooey into the water like that thats doing it.. .Unless you accidentally tracked a substance in on yourself when putting it in the tank.
You know, I'm not a fan. They are nasty stingers to me personally (break out the vinegar!), I've had so many people tell me theirs ate their fish, the concept of this moving on its own scares me, but seeing that color green in your tank really, really makes me wish I had one.. almost. If it werent for it just being a skinny 55 I'd really consider it.
Maybe when I have a bigger tank I'll save up and hunt down a red one. But dang, you actually have me thinking on a carpet, that takes a lot. Color me jealous!