Need help identifying what’s on my sand bed.

[Greg Depo]

So I need help figuring out what’s consistently growing only on my sand bed! It’s almost a light neon green and it comes off almost as a mat. It comes off in almost sheets. I clean the top layer of the bed and it’s back a few days later. I’ve included some photos (after I removed most of it) What do you guys think?
Tank is a little over a year old. Nitrate looks like 15ppm and nitrites are at virtually 0. P04 is at .25ppm. Thanks!