So, I ran into this challenge like 8 years ago. Long story, I did a 35% H2O2 bath that was diluted to maybe 12% in a Brute (maybe 4x) followed by multiple SW WC's for a few weeks. Maybe a WC a week, but it has been too long.
The amount of gunk that came off was insane and in hindsight of recent events, I probably should not have done it in the summer in a hot garage since I had a ton of zoas on those rocks... So, be careful to keep the activity outside or at least in a really good ventilated area.
In my case, I bought a used 46G Bow with a 10G wet/dry that I had converted to a sump initially and then a DYI algae scrubber to try fighting the battle. The seller likely sold it because he gave up on the fight to algae and pets. He had (or I later introduced) grape cap, Halimeda, bubble algae, HA, red and white flatworms, a gazillion pods/amps, spiders, etc.
I will say that everything (softies, frogspawn, hammer, etc) all looked a lot healthier than my current tank. I can't find a full tank shot atm, but you can see a top down of the macro and HA.
I restarted into the hobby and re-used that same rock that has been soaking for 8 years. Rock is prestine, almost no algae in the tank, but I will admit that I am running the tank more like an SPS tank without any real SPS or dosing regiments. I just don't want to new tank to ever get to that place.