Hard to tell with how blurry it is. I’d say it’s a type of algae, probably hair algae. You can scrub it off with a toothbrush while doing a water change (doing your best to siphon all the debris that comes off) and it’ll be under control.
Alternatively, it could be colonial hydrozoas. Though not uncommon, large numbers can irritate corals. You can also control their numbers with the method suggested above. Both thrive off of excess nutrients and phosphates, so if they do start spreading, reduced feeding and increased water changes could help. I would highly recommend getting in the habit of dipping corals and scrubbing plugs before adding them to your tank; this will save you a ton of trouble in the future!
Hair algae loves the frag plugs. You can kill algae with hydrogen peroxide, but it can kill the coral if you're not careful. Do some research before attempting it.