Im also with Jose and Darrel.
And an established algae outbreak like that is going to cost you some loss anyway.. No matter how you approach it you will lose parts of plants. Using h2o2 on moss will likely damage the moss as much as just cutting the infected parts away. The downside with peroxide is you never know the result till you see it, you could be lucky some mosses do good others are killed rather fast. Try some on a small part first and how it takes it, and that is still not a garanty the other parts would take it as well. I killed a lot of my precious moss with it and wouldn't advise to use peroxide on it. Rather leave to moss to an army of shrimps and see how it goes, if nesecairy help a bit with sciccors.
Even with a black out you still have to cut away the plant parts infected. When algae grows on a plant it attaches in the pores and cells of the plant, that leave is damaged anyway and get necrotic edges where the algae was and most likely to die over time. Green spot is less agresive than the beard and hair family when attacking leaves.
With certain plant species a longer term blackout can trigger some melting but rarely will kill the whole plant. But i came to find out that peroxide also damages the plant a bit, some plants take it better than others. If you do a peroxide treat and a blackout after that there certainly is going to be some melting, because you take away the plants energy supply to recover. I tested it on some rotala's and all older leaves did melt off after the peroxide and black out. That was just a curiousity test, i didn't do a blackout for my tank, but wasn't also that vastly infected yet before taking measures.
Cut away heavily affected leaves and try the blackout first and see what's left after that. After that you could spot treat hardware and substrate with peroxide. Do frequent 25% water changes, extra filter cleaning and substrate vacuming 2 or 3 times a week. Inspect the bucket what comes out, keep the cleaning regime up till you find no more algae strains floating around in the bucket. Then go back to just WC once a week with a good firt regime and a steady co2 supply.
You already did down the light periode, now you could add floating vegitation like Salvinia and or duckweed or both to reduce the light intensity. you could also dim the light but floaters are more sufficient in the way that they help combating the algae after all the cutting you did. They floaters get a lot of light at the top and so will be triggered to grow fast, their roots will suck up the nutrients while your substrate plants get nutrients as well while slowly recovering in the floaters provided shade. You could throw in a hand full of Egeria densa as wel and just let it float for the time being, till your substrate plants have come back and show new lush clean growth you can take that egeria out again. Buy an army of shrimps like Japonica not the most beautifull but seem to be one of the best algae combaters and don't feed them to much cucumber etc. They eat almost everything, if you feed the fish froozen Dapnhia or bloodworm the shrimps also will snack on that they even snack on dry food. If you provide them with to much extra vegtable treats they will ignore certain algae. Hunger makes raw beans sweet.
You can spoil them with treats when the work is done. They stript al what was left of my slowest growing ever seen bolbitis diformis from staghorn. while reading a lot of articals where is stated that they don't.
There are many other strategies working as sufficient as you will find here, but this is the way i got around it. By the help of UKAPS members provided information. Just read articles, replies and filter out what suits your feelings and situation the best.
Good luck.
