It does look like a ball Nem which I would consider a pest because it can spread and sting your corals. If you are able to remove the rock, just scrape it off with a flat head screwdriver. If you are unable to remove it, go to walmart and get (mrs. Wages pickling lime) aka kalk and mix up a ratio 50/50 with ro di water. Shake in a small bottle until its throughly mix and squirt right on top of the nem making sure to cover the entire top disk. Turning off the flow will allow the kalk paste to remain on the nem for longer. Try to avoid getting any on your corals.
Avoid using more than you have to because it will change your ph at some point. I like it because its just kalk which your reef will benefit from