It look like it's growing on it. It even looks like some of it is on the plug. (see around 9 O'clock on the plug in the photo)I've never seen that before. Does it look like something around the zoa or the actual zoa tissue?
Really?? What makes you say this? Is this what they leave behind? Any ideas on what to do?flatworm
I think it could be a flatworm, not sure, how long has it been there.
Have you tried giving it a squirt of water to brush it off?