you can google Zoa pox, which are white lesions on zoanthids and palythoas. They start small on the stalks, generally towards the base, and gradually work their way towards the head of the zoa or paly. The zoa or paly will then close up due to irritation, and will eventually recede and die as the ZoaPox covers the entire head.
there are a couple of method to cure them, works fine,but have to take action early. it can be spread to others in your tank.
keep monitoring it as it can be normal if still looks fat fat, but it may melt off from then.
my own experience, lighting can help but not the cause.
i got many frags on sand base.
but system different can be various cases