Spinoid worm, or barnacle.
How old is the coral(how long have you had it)? I would say barnacle if it'sd a recent addition as they typically do not live long in our tanks.
Neither are harmful to that particular coral.
I have several acro's that have vermetids within their structure. As the coral grows, the vermetids grow with it. It's ugly seeing the little spires with acro flesh on them, but it does not hurt the coral one bit.