They look like a type of bryozoan. If they are slightly hard and break off, then more than likely what they are. If they are soft and retract when touched, they would be colonial hydroids. Colonial hydroids can irritate corals, so best to check for retraction.
If bryozoans, they are harmless filter feeders and similar to moss, for example. If you want to remove them, no issues there. Just scrape off into a net and discard.
If hydroids, they have to be starved out or removed via a little manual effort. You will need to remove coral from tank, place in a Tupperware container with some old tank water. Scrub the Skelton, being careful not to scrub the flesh of the coral and then remove the coral from the tub for enough time to either put superglue gel over the entire Skelton or apply a 3% peroxide to the skeleton via a cotton ball. Again, make sure not to get any on the flesh of the coral. After that, you can rinse the Skelton in another tub of old tank water and place back in the tank.
To starve them out, you will need to direct flow right at them, but that may be too much flow for the coral, so may not be an option.
