Same thing happened to one of my chalices recently while the outside seemed perfectly fine, I'm Not sure what caused it. Id dip it, feed it, test kh, ca, mg, and do a decent sized water change. If it doesn't stop or begin healing; in my experience when a colony starts receding in the middle it is a slow recede. Eventually you could take it to LFS and have them cut it up for you possibly then you'd have several smaller pieces that haven't been infected.