Protopalythoa sp. (large button polyps)
I have 2 types of them (one set has polyps 1/4" diameter - the other larger at 3/4", and both are very hardy, almost hard to kill - but also rather aggressive - so keep all other corals 2-3 inches away. Mine have very bright green centers, but
do not seem to reproduce very fast - so I keep them....some Protopalythoa sp. can reproduce rapidly in a reef system and damage/overtake other species...
It'd be best to research more your own as you can - what you have before they become a problem - also see "palytoxin" as all of the Palythoa genus produce this toxin - and if you attempt to remove these polyps - use thick rubber gloves & goggels - and for Gods sake - don't damage them/scrape them in a tank with fish in it - a mass release of this toxin in your water could wipe out all your fishes! ! !
Submit yours to some light for a month or 2 and see if the centers take on some green and look for rapid growth by division....usually the more "bland or tan" they are - the more rapid growing. Most specific species of this type are still unclassified.
Here's mine (3/4" dia type) [I think is Protopalythoa mutuki]: