I had that on my borealis before. Never found out what it was. I tried to pop/rub off, but the flesh just tears away. It's like they are filled with water. I just left it be and a couple of months it just went away. It was only on a couple of branches and the only coral affected by this. I thought it might be some kind of bacterial infection, but nobody ever gave a real reason/answer. Good luck and keep an eye on it!
To be honest it looks to be taking on a new growth pattern, is it growing into strong flow as it fills out?? If you see tissue receeding i would worry but to me it looks like new growth, just more stout, it appears healthy from the pic.
I had that on my green slimer. It wasn't on every tip just a couple of the "v" sections where it was branching. It eventually popped and was like a little crater. It grew over it and was fine after that. I thought what Mev did that it was an infection of some sort.
if the tissue starts peeling off you need to worry...but in most cases RTN and STN, it starts from the base and works it way up...i would quarentine the coral if the flesh does start peeling so it dont start a chain reaction and wipe out others around it....but for now just keep a eye on it....i only saw that one time on a acro i have and it was where it fell and got stung from another one....hth...