are there any areas sheltered by hardscape or a large-leafed plant in your tank? Just so you can try and determine whether it's something precipitating out of the water column, or if it's being produced by the substrate.
I have a current tank with MGOCPM, and a flourite sand cap, and I haven't seen anything like that, also had one for about a year with MGOCPM and black flourite cap, and didn't notice anything similar to what you have.
Any other noticeable traits? powdery, fibrous, goopy or anything?
I just reread your post, and noticed you said it's only been filled for a couple months. I wonder if it could be related to the extended dry-start, and only being recently filled?