I’m more perplexed by the giant sand holding cube

Looks like algae to me. You could try an algaecide, which you can pick up at any aquarium supply store, but that may just turn it brown. Chances are you're eventually going to need to scrub that off the acrylic.

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

Hi all! I’m trying to identify what’s growing and how to get rid of it since we’ll have students digging around in the water and play sand. I’ve tried adding pool shock but as you can see it didn’t work, maybe bc the shocked water didn’t penetrate the sand. If you can point me to a subreddit that may have experience with something like this or if you have any info that may help would be much appreciated!

Soilent Green!