Identifying and treating acan disease

[brixle]

I've had this acanthastrea echinata for a bit, and I've noticed that on two sides it looks like something's eating away at it from the inside. I've tried painting the affected areas with Seachem reef dip twice over the past week. It seems to have slowed it down, hopefully it'll stop it altogether.

The mushroom in the picture isn't next to the acan and isn't stinging it, I just moved it there for the photo. Does anyone have any idea what's going on here?

[rustyfox9]

To better understand the issue, could you share your tank parameters. It's possible something nearby is causing the damage, as the affected area does resemble a sting.