That looks like a limpet judging from the pictures. I had one on my pink hammer coincidently some time ago. They aren’t really a problem until they start digging into the hammers flesh. If you want to get rid of it just do a peroxide dip. That is what killed mine. For next time though I would consider putting this on the “LPS” forums instead of the “Soft coral” forums as Hammer corals are LPS.
Edit: I don’t think it’s limpit. I forgot the actually name for it, so if anyone comes along with the name of it then feel free to correct me.
Treatment is still the same…