Yea look into a lux meter $15 on
Amazon. Look at my threads for lux meter upgrade.
The SB will fry stuff. Chinese B too.
An SB at 12 in is capable of 1200 to 2000 par. That will kill coral. And plants too like corraline. When you blow off the algae is there any corraline underneath? I'm betting not as the corraline is likely dead and the cyano is just feeding on it.
An SB at 18in will deliver about 1000 par.
You really don't need more light.
It's about balancing slowly the nutrient uptake of the coral with the light it need to do it. In short enough food in the water and enough light to let it eat it.
Flow is a whole othe factor.
There really is something about a mature tank too. Acro are pretty sensitive. All tanks are different. Some hav an ugly phase for months some have none. Some for a long time.
Large bacterial poulations may have something to do with this as well as phosphate content on the rock plus the micro fauna as this may be feeding the the stony corals as well. I asked rand farley this. That was his take.
With high nitrate there's a lot of extra food in the system. Most or many will GFO or chem clean it or totally over skim.
As in a new tank with dry rock it's the bio filter still gaining strength and not processing well yet. It's rock. It takes time to penetratit.
It's a reason I'm a fan of refugium. It's two biological filters and they kinda balance each other.
Looking at the corals you have bleaching and browning. That's not normal.