That looks like bryopsis. Be careful when you pull it out. It is like aptasia in the sense that it multiplies when you mess with it. Pieces come off and attach in other parts of your tank.
Use Kent Tech M to raise your magnesium up until the bryopsis turns whitish. When it turns whitish, pull it out as it is dying/dead.
You have to use this specific product as there's metals in it that kill the bryopsis.
I had a bad feeling this was no good. I had it on a frag plug and peroxide dip seems to have worked for it the rest is on 2 rocks that I can easily pull out should I try that? Ill look up the Mag raise also thank you!!!