WHAT is this Stuff?...

I'm sorry I haven't chimed in but probably because I have no idea what that is I would say since it's only on the eggcrate that it makes sense that that is leeching something causing it. It's weird though since a lot of people keep egg crate in their tanks although it's usually under the sand, perhaps it's the light being right there that is making it bloom??

 

Thanks, I cleaned one area of the squares by hand with something that looks like a test tube cleaner, made sure my Mg was high, used a bit less light on the frag rack, and where I took it off it seems to be staying away now.

Luckily- it's the only area I have it is on that new frag area/black eggcrate.