edit: Bradleym beat me to the ID, but agreed majano anemones was my first guess.
edit #2: There’s a device called the majano wand that zaps them but can’t access some aiptasia in cramped tanks. Aiptasia X claims to also work, but I’ve only used it for aiptasia and not majanos. If using aiptasia X make sure to turn off flow for about 30 minutes and also apply the aiptasiaX super slowly out of the syringe so that the anemone wraps it’s tentacles on the syringe and eats it up.