Got it figured out thanks to my dude
@Blue Demp ! Turns out, the RODI unit was producing water with a TDS of around 71ppm. (That's bad...)
I replaced every filter on the unit and re-measured, now it's producing 0ppm TDS. That would explain the algae outbreak after every water change.
I've had the tank blacked out for 4 days, going on 5 and the sand is back to being beautifully white again. Not going to rush things, but I'll say that I'm thrilled to have figured out what seems to be my issue. I'll post pictures after the blackout is done tomorrow.