[White Matter]

I’d normally agree but I’ve been vacuuming regular and it’s growing across substrate. I had even vacuumed up much of the gravel and replaced it and within a week it was all grey again. 

[White Matter]

I have a white matter growing on my substrate. I try to vacuum it up but keeps coming back any idea as to what this is?  My tank is well established and no issue with ammonia/nitrite or nitrates.