Your alkalinity should be fine if you can keep it there. The corals IMO aren't that bad looking. I had a "red millepora" that under my lighting (400 watt Radium with VHO Actinic) shifted to a purplish color within 6 months. It still grows fine. You would be amazed at how much the lighting spectrum can affect a coral's color within a few months.
There can be a few things happening here. Even though you have low nutrients, algae can still come around from time to time. I once had a frag with a touch of hair algae on the plug and didn't bother to properly remove it before placing it in my tank. Within a few weeks I had a slight algae bloom even though I test undetectable with nitrate and phosphate. After a few good water changes it simply starved out and hasn't re=appeared. I think too many times we attribute too many problems to phosphate. Personally I think you need more fish to feed. For me, because I carbon dose I must feed or my corals will go pale. When I wasn't feeding enough my coral polyps would close and colors would fade.
Currently in my 72 x 18 x 27 150 gallon SPS tank I have a Kole Tang, Yellow Tang, Vlamingi Tang, Emperor Angelfish, Coral Beauty, 2 Lyretail Anthias, 4 Blue Chromis, skunk clownfish, sixline wrasse and a yellow tailed Blue devil damsel.
I dose 3.5 ml. of vodka per day, MB7 low nutrient system dosage per directions once per week, change 25 gallons of water bi-weekly and feed the following:
8:00 am feeding before work: 2 cubes Mysis, pinch of Omega One Marine Pellet
6:00 pm 1 cube Mysis, 1 pinch Formula Two Marine Pellet
9:00 pm (before lights go out) 1 cube Bloodworms, 1 pinch Omega One Marine Pellet
Before I added the Emperor and Vlamingi tangs I wasn't even feeding half of this and the corals began going pale and the "peach fuzz" like you have on your rocks was abundant. I attributed all of it to phosphate so I slowly fed less and the corals lost polyp extension and began going pale. Each month I ramped up the food amounts. If I had excess nutrients then clearly the corals would begin looking worse and worse. After 2 weeks of adding more fish and fedding more my skimmer was more productive and the coral color and polyp extension returned. Every week I can see the colors getting better and better. From now on I will continue to add small amounts to each feeding until I see where the limit/threshold is before I detect organics building up.
I think too many of us feed less for fear of nutrients when in actuality we should feed more until we see where the limit lies before we see scale back. Our fish are much more plump in the wild than in most captive systems. I strive for full bellies and happy fish that look colorful and healthy. I would add one more light feeding to your tank and see how it goes. You may be surprised.
It's hard to see but they are 100% neomeris annulata. When they are small, they look exactly like hair algae but once they grow to 3/10", the difference is clear. I wish they are hair algae because it would be easier to take care of then this crap from hell. I was told neomeris annulata only drives in SPS-like environment but I don't know if that's true or not.
