Looks like dinos to me. Curious how old the tank is, what type of rock (was it dry?), and do you have sand?
Tank is about 9 months old, I've probably had the problem for at least a month or 2, but I was too busy to really focus on anything and at first I thought it was Cyano but all the steps I did to lower nutrients to starve Cyano did nothing, hence me believing its Dino's.
I used Real Reef Rock, but it was pretty dry when I started the tank. I do have sand, but only about an inch of depth. All my fish are doing good (angels, PB Tang, Royal gramma, Leopard Wrasse), and so are maxi mini nems, but corals are suffering.
Are your hermit crabs and snails dying off?
Sounds like you're on the right track. If you mix your own water be sure that is 0 TDS since dinos thrive on silicates.
If it starts to drown out coral, manual removal followed by a 3 day complete blackout has done wonders for me in the past.
Every "new" system seems to go through this a few months to a year in. Really until I develop other algae that can compete with it. If you run a refugium you May need to hit it with a blackout to knock it back and allow your macro algae to start and compete with it. Just my experience with it.
A great test that some posted here:
Siphon algae through a filter sock into a white bucket. Place the bucket outside in direct sunlight. The dinos should start to regroup and form some snot within a few hours/day. Probably done best on a warmer day but a cool test to confirm what you are dealing with.
I have had some snails die recently, but others seem to be surviving ok.
I have an RODI unit with 2 DI stages. My TDS into my DI is under 10, and my TDS after DI is 0.
I do a manual removal via siphoning when doing water changes. I haven't done a blackout yet, but may try it.
I originally had a fuge, but the chaeto and other algaes got smothered and we're dying so I pulled it out. I got some coraline algae from Ali that will hopefully grow soon once the Dino's die out.