The best way to get to that algae is to remove the rock because its worth saving your tank from it but if you just can't, try and muster up just one 75% water change that will emerse the rock and let you spot treat it.
no harm will come to anything on your rock from short emersion and treatment, I leave my delicate pico reefs empty for up to ~8 minutes sometimes doing cleaning work.
In nature, they can be exposed for up to 8 hours so the reduction to just minutes is well within the adaptive ability of 95% of the corals we keep.
Just know that when you can get to the rock out of water somehow and put a tiny bit of peroxide on it and let it sit for two minutes then refill, it will be gone by next day. There's not really a big hurry to treat it with this option in your back pocket...whenever you treat it, its gone the next day. have fun its the best cheat ever!
There will be no use for tangs with occasional spot treatment.
Matter is matter, no matter what you do with it in a tank. People think fish eat algae and make no waste of it, but they'll tell you a spot treatment doesn't get to the root cause of the algae. Its a circular argument, fish cause more algae so in that sense anyone recommending you a fish for algae control is making things worse.
Keeping your tank low in nutrients by focusing on coral and inverts and first and fish bioload last is what you want to do, trust me you are lucky your tank is in the building stages.
There is no stage that algae or diatoms should be present in your tank, what people currently advise about algae is the reason it becomes a problem for most people. You do not let a sporulating organism + a self regenerative organism develop biomass in your tank, unless you want them hanging around.
The old untrue method of letting any algae get a foothold in your tank allows the crevices and interstices of the rock to catch and hold algae spores and regenerative biomass that can give you a headache for months to come.
The new way of algae control is manual gardening, if your clean up crew is working then you'll never have to do it. But if there is algae, or diatoms, or anything else that could wreck your tank you remove it during a water change, its never allowed to stay.
there are other simple approaches I've used to control algae and other pests like aiptasia-- cigarette lighters, just burn the organism you don't want if its on a rock you can lift out of the tank. Its harmless, flame denatures all proteins there is no poison from burning thats another statement made without knowing the actuality of the practice.
When I got the usual brown diatoms during new tank setup, I siphoned them out and replaced any top layers of sand they may have started on. I wiped it off the glass at each water change, then did large water changes to export it out of the tank (contrary to the 1990's opinion that only small water changes are safe)
So, if you do everything backwards of what we've been told the last 15 years, your tank will look pristine for years, that's the real deal. Manual gardening commands a tank to run and look like you want it, the best way to run a reef. By all means use grazers if you can settle on some, just be prepared to jump into action when they fail, do not let dangerous biomass collect and it won't wipe out your tank, regardless of perfect water parameters.
Having incredibly low nitrate and phosphate water isn't the only way to beat algae, in fact its unnecessary. Its outdated, a result of very few pressing ahead with new approaches and just rehashing old info.
If old info worked, we wouldn't have algae problem threads but there are thousands on every board. We needed something new in the game to keep these unnatural tanks looking natural.
hope ya'll took tylenol before reading this lol its 90 wpm at work