ID Please! Dark gray slug on cynarina

[jassermd]

So yesterday evening, I noticed my cynarina not acting quite normal... mind you, this is a large cynarina with a fairly wide skirt (that's what my wife calls it). It sits in the sand bed and has since day one; Routinely open wide and happy as can be. Yesterday, it was retracted on one side and not so happy...
This guy has been in my tank for 4 months and was dipped prior to going into DT. These were not there then, that I can assure you!

Pulled it to dip and noticed 2 gray like slugs on the skeleton. They didn't move when poked and were quite stuck to the skeleton.
First dip was Coral Rx, second in iodine.
I managed to get them off the skeleton and it appeared as if they were "digging in". They weren't difficult to remove, kind of squishy, but wouldn't come off in one piece.
I took pics while in the dip, but not after. For an after shot, just imagine them not there and some superficial skeleton eroded
First pic is the coral normally, fully open and happy for reference.

Anyone have any idea what these are?

 

[SauceyReef]

Looks like a sponge to me.

 

[dedragon]

i would say a sponge as well, have you tried to remove it with a knife? or any after pics from removing them

 

[jassermd]

I removed them both with a scalpel. Forgot to take pics after, but they were stuck quite well and some skeleton (<1mm depth) underneath them was exposed (typical CaCO3 matrix)
Dips did not do anything to them, hence the scalpel... When in doubt, take it out!

 

[dedragon]

agreed, good move. If it comes back after using the scalpel maybe dry the area and dab with a bit of hydrogen peroxide using a qtip, to try to kill what ever it is for good.