Whats on my acro?

[X-37B]

I have 2 wild acros that have this blue slime growing around the base. I remove the coral and scrub it off. It has not came back on 1 but just showed up again on the last one I cleaned a few weeks ago.
The isssue is if I leave it it eats the acro from the base.
It comes off easy and is slimey.
Anyone know what this is?
It showed up a year after the corals were put in the system.
It does not seem to effect any others.
Pic with flasjilght so you can see the blue around the base.
This coral had growth down to the base. Coralline has covered the area I scraped. Zoom in on the base. At first I thought sponge but its not like any sponge I have seen.

 

[Tahoe61]

Sure looks like a sponge. I believe I see Osculum.
Superglue would probably stop in it is tracks.

 

[saltwaterfish.com Verified Sponsor]

2nd vote for sponge, super glue would work well or some people do coral dip, but this does not work on some stubborn sponges.

 

[X-37B]

So do sponge encroach on coral? Never had that before. It kills the coral slowly growing up the stalk of the said coral.
When removed its like slime from an acro.
I will clean again and apply some putty and try to seal it.