Looks very abnormal. Almost like a cyano on your bubble, a parasite perhaps?
Look closely and see if there are creatures on it or if it is just brown slime/spots.
Polyp extension shows that your params are probably good but I have the exact same bubble you do and have never noticed this happening.
Please post your parameters and closely inspect the coral to try and see if it is discoloration or if there is actually something on the tissue.
Also, at night inspect the skeleton with a flashlight and let us know how it looks, post a picture of that if you can also.
If this keeps up the tissue may fall off of the skeleton and possibly die.
How long have you noticed this? Does it come and go?