So whats going on here?

[Nano]

Ok so I heard a lot of people saying to try birdsnest for my first sps, so I found a $10 green frag and figured, what the hell, the coppers gone, levels are back to normal whats $10? So I bought it and its polyps are all out and looks beautiful, I turned down my AI a bit to acclimate it over a few weeks, but this morning it has some sort of stringy gunk on it, please tell me this isnt RTN or something bad, if it is, it is.. if not what is it?