Glass anemone. For sure. I'd check into it. I mean not to argue or anything. Low light. Spread quickly. Considered a nuisance. Powerful sting. Wil sting and kill corals. In no time it will cover the tank in offspring. Careful in it's removal as it's easy to cause it to reproduce while removing. Leaving a trace amount of it will cause it to grow back with a vengeance.
If peppermint shrimp isn't an option, there is an easy way to get rid of it without it spreading, if you are sure you get all of them at once.
Mix Kalkwasser with some tap water to form a thick milky liquid. Using a syringe and needle, inject this solution into the mouth of the anemone. The only trick is to get it into the internal organs before the anemone shrinks itself too closed to access it. If you're on target, it will shrivel up and die within 24 hrs.
This one appears to be on the bare glass bottom, so shouldn't be too hard to get. Watch the rest of your tank closely, and if you see more, then I would consider the peppermint shrimp to be sure you get them all.
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