Hey Jeremy,
I feel your pain and these issues suck. I'm dealing with a nasty lobophora algae problem that I'd gladly trade places with you.
As for your issue, I'm gong to repeat a couple of important questions John asked that you may have missed.
walk us through your filtration.
- what are you using for mechanical filtration? Filter socks, filter floss etc
- what are you running for chemical filtration? GFO , gac , bio pellets etc
- how is the chemical filtration being used? Reactors, fluval, filter bags etc
- what are you running for biological filtration? Live rock, bio balls, matrix, siporax etc
- where did you get your live rock? What type is it?
- how often and what percent water changes do you do?
- have you tried any chemical remedies like algae fix marine?
Without me knowing any of that and based on your years in the hobby, my hunch is that the possible source of your issue is either your water or your rock. The first thing I would do is test your RODI water for phosphates. I would test it at least 3 times due to the range of inaccuracy of our hobby grade tests can produce zero phosphates even when some are present. If that passes that would lead me to believe your rock is leaching phosphates. There is a hole host of reasons why that could be happening. Most likely they may have always been present, but I've also read that phosphate free rocks can absorb phosphate and then when they hit their saturation point they begin to leach it back out. Bottom line, you need to find the source of your phosphate. Short of that, the alternative is to combat it with GFO or outcompete it with an algae scrubber or fuge full of chaeto.
As mentioned previously, 99pct of the folks posting algae problems get readings of 0 n and p. That doesn't mean you don't have an n and p problem, it just means the algae is consuming it. That is mostly why initially no one is taking you up on the offer to come to your house and test chems. If after some board discussion you still have issues, then I'd be happy to pop over and take a look to see what we are missing.
Good luck man and have patience with us as we ask questions. I know it's frustrating, but we want to help you.
Edit: forgot to mention, I would also strongly consider sending your water out to TRITON to be tested. It's 50 bucks, but gives you a pretty comprehensive snapshot of your water parameters. I did it about 18 months ago and planned on doing it periodically. I may do it again very soon. Iron is another biggy that is critically important for algae growth.
Best,
Mike