Nuisance algae ID and treatment - help please

[Klwheat]

Ok, not sure if this is the right thread (please feel free to move if there’s a better one) but figured I’d try and get some expert advice.
I have a red “algae” that started growing in my tank a few weeks before I got back from Afghanistan about 6 months ago. I have no idea what it is, but it has been killing and/or growing over multiple sps and lps corals.
It doesn’t appear to be cyano (no bubbles) and didn’t really respond to chemiclean.
Tank parameters are:
Phos- 0.0-0.02 (gfo changed about every 4-6 weeks whenever phos shows up)
Nitrate - 0 (has been 0 for as long as I can remember)
Alk- always right around 8 (7.8-8.2 or so)
Calcium - 420-450
Mag - 1250-1350
PH - 7.9-8.2 throughout the day
Temp - 77.5-78
I have tried feeding extra (still no phos or nitrate), feeding less, less light, more light, more water changes (every 2 weeks) and less water changes (every 4-6 weeks). All with no change.
Tank is a 180 mixed reef, lighting is 3 Radion gen4, fuge underneath that grows a decent amount of chaeto lit with a gen1 Radion (on red/blue), good sized skimmer, and filter socks changed weekly.
Tank has been set up for a little over 2 years. All corals have grown great, until this happened. Now, all corals are growing great except the ones this stuff hits (pics attached). 2 of the corals shown are a green birds nest (almost 12”) and a stylophora (almost 8”) that we’ve had for several years (from previous tank).
Feeding is pellets daily (Neptune crossover diet) frozen most days (homemade mixture), coral food and trace elements at night most nights.
Any help to ID and/or get rid of this would be great!! If there’s any more info that would help y’all please let me know.
Thanks in advance!!

 

[Klwheat]

Any thought from the experts?

 

[Flippers4pups]

Looks like cyanobacteria to me, but here's a test you can do to verify:

In fact most of the time it's another form called spirulina.
Absolutely 2 different forms of bacteria with 2 different options of treatment.
So let's get onto finding the positive identification of the bacterium.

Take a portion of that red mat you see in your tank and place it into a cup with about 2 cups of tank water.
Now add 1ml of 3% hydrogen peroxide
Over the course of several hours you may start to see a change in the water and the color of your sample.
If in fact it is the common cyanobacteria and not spirulina the water will start to turn a pink color and the sample with start to turn a green color.
Now if it is spirulina there will be no change as h2o2 has little effect on the individual cells.

Chemclean should work on both.

 

[Klwheat]

Ok, followed your directions for peroxide test. Pics below (start and 4 hours later) but after almost 4 hours...no change.

 

[Flippers4pups]

It's most likely spirulina then or it's a red algae. Chemiclean should work if it was cyanobacteria or spirulina ,but you said it didn't have any effect.

Let's get some more eyes on this!

#resfsquad

 

[Klwheat]

It originally seemed as though the chemiclean made a dent...but didn’t last long