Acan Problem/Disease

[JDT Reef]

Hi....

I have this acanthastrea echinata for some time and noticed that on two sides it appears that something is eating it from the inside. I directly painted the areas with Seachem reef dip twice in the past week. It may have slowed the progression... hopefully stop it. Time will tell. Anyone have any idea on what this is?

The mushroom in the picture is not next to the acan and is not stinging it. I just moved it there to take the photo.

 

[aromano]

what are the tank parameters? Anything possible near it does look like it was stung by something.