I'm not willing to call it zoa eating vs. monti eating from that picture, but it doesn't look like an aiptasia eating one. I would remove it and start a frequent search for more along with their eggs. Don't crush it by hand (palytoxin is real), suck it out with airline tubing or a cut off pipette.Additional edit, while I still don't think it looks like an aiptasia eating one (berghia), is it safe to assume that you have not intentionally added any berghia lately?