White patch on my chalice coral

[Spydersweb]

Hello all,
So, this morning I noticed a white patch right in the middle of my chalice coral. It seems to be a little indented. It couldn’t have been there more than a day or two. I’ve had it for almost 4 years and it’s done perfectly fine. I haven’t made any recent changes to the aquarium, and all of my other corals(hammers, Duncan’s, blastos) are nice and happy. Is anyone familiar with what this might be?

 

[nereefpat]

Bump.

Perhaps some salt creep fell on it? I would think that should heal up.