It's hard for me to tell from the picture, my eyes aren't what they used to be, but to me they look like young aiptasia. There are a variety of things out there that make outrageous claims and some do work some of the time but for the most part the only thing that I've seen work well for large infestations is berghia nudibrachs. They're obligatory aiptasia eaters and do a heck of a job. Once the job is done you can move them on to a new home, there's always a nearby reefer in need of an aiptaisa killer. You could also try a raccoon butterfly, aiptasia eating file fish or copper banded butterfly, but there's no guarantee that either of those won't go after coral once they've cleaned up the aiptasia. Of the three, I've personally seen raccoon butterfly fish eat aiptasia like they're never going to see food again.