Got a couple thoughts on this. Flatworms could be hanging around somewhere on this guy or STN. Either way, give it a good dip!If it doesn't seem to help or the white spot keeps growing, I would personally try and frag it.
I don’t think that’s AEFW bites or damage. I’ve had something similar happen to a BC honeycomb frag recently, and I just watched it. Sure enough, tissue is growing back around that area. Point being, sometimes a little damage can be monitored before you do anything.