Thats Aiptasia, they are considered Anemones (but they're the kind you don't want in your tank at all) They are an invasive species and they can duplicate in quantity if left untreated. They can sting your other corals and fish. There are products or fish that eat aiptasia like an filefish. Hope this reply helped!
I am not very knowledgeable in this department, but I do agree with cell, it looks more like a paly to me than aptasia. Hopefully someone else will come in and give an opinion.
Like others have said, it's a palythoa. I would remove them as they will take over your entire tank. As mentioned before, search up palytoxin before handling them. You will hear many people argue that you don't have to worry about it, and while that may be true in some cases, it's not worth the risk. Do your research, but typical PPE should be goggles and gloves. Make sure the room is well ventilated, and if you are getting rid of them make sure to dispose of the properly. Wash your hands after too.