Yeah looks like dinos to me. Your 3 day blackout should be total darkness. If you blackout (tape back trashbags up) the glass and let no light in, including ambient, your tank will look spotless on the 4th day. Doesn't mean it's dead in my experience, it can come back.
It's toxic, including to fish, so skim very wet while blacked out to remove as much as possible.
4th day actinic lighting only. From there slowly ramp back up to a normal lighting cycle.
With it getting worse after a water change I'd look into your water source. They thrive off silicates. I like to run SeaGel in place of just carbon during and after the blackout.
I've read that you can kill dinos by raising your pH to 8.6-8.7 constantly for several months. I haven't tried this yet. There is also Algae-X that seems to have great reviews.
Good luck in the battle. Hopefully it didn't transfer to your other system. I'll be tagging along.
I really like the scape and look forward to seeing your results with Zeovit once the dinos are gone.