I had a live bottom fish tank with a dense sand bottom, (which was a mistake, but not relevant now), and ran into this and other colors of gunk growing against the glass where the sunlight hit it. It could be algae, but given that the sand is probably excluding the air in your cube, like the quartz sand in my tank did, I would hazard a guess it could be cyanobacteria as some can live through anoxic conditions. I didn't keep the bottom much longer to find out definitively, but I've read hydrogen peroxide added can kill it, but getting out of the sand will probably require you to stir up the bottom. If it's not you big of an issue, staring over with the pool treatment from the get go would be the best solution