DKoKoMAN’s 305g Basement Reef

[DKoKoMan]

I’m just wondering if anyone is able to help me identify this white (almost a feather pattern) thing that is on the corner of my glass. It literally appeared over night. My guess is maybe some sort of eggs got laid... is it good or bad? This is the second time I have saw this, the first time was in the opposite corner and when I was using my magnet it all detached. Any help is much appreciated! . I have attached a photo (hard to get a great photo).