ok, you have 2 ways I think, if isn't retracting at all, can be just algae, which harm the CC when it feed and extend the tentacles, and the mucus to be from the CC.
if you wish, but you have to be very careful:
1. take eyelashes tweezers (it's flat) and try to reach the coral tissue and grab the think, pull it out, probably the CC will heal it self. (or someone else more precise can do it for you ?) Just don't push too much in the tissue to grab more, and that way to damage the CC.
2. not sure how you can use the super glue, if you can or someone ole for you: spill a bit of liquid super glue on a glass or something with hard surface (like a porcelain dish, to can scrub it later to clean it), with needle take small amount and taking the CC out of the water, put some glue in the hole, do that few times until you see everything in the hole is sealed, no the hole to be completely filled with glue. I'm not sure can it overgrow it later on but for sure will eradicate the CC for a while, after the coral isn't going to be bothered about.
If I'm I'll go with option 1 and later the CC will heal the area.
Just whatever you do, be careful.
If isn't retracting at all isn't vermetid, or any worm/snail in, it's more like hair algae grow on the skeleton (the upper side), when at some point the CC is damaged (dropped on the bottom, or hitted/scratched by hard object - turbo snail snail, LR moved near it (that's examples)
so many varieties too
