Yeah it's really transparent but I have about 3. It's interesting that after my tank running for about 3 years now that they are just now being seen. I'll look that up though. As long as it's not something bad then I am happy
Well it doesnt look much like the cup coral /: and it doesn't have a stem to it but it looks a little more like the hydroid . I'll look again once the lights are out
Looks a little like strawberry anemones or Pseudocorynactis. If it is, they are for the most part harmless. They much prefer colder water, and less light, but have had them in my tanks before, mostly under rocks, and in shaded areas.
Probably anything with an extreme ph should kill it. So aphasia x, lemon juice, kawlkwasser paste, citric acid. Or if you have any of the following at home: 3w laser or majano wand.