Would you say that maybe someone is over feeding the tank? Or maybe adding a little extra fertilizers each week?
Most likely a type of cyanobacteria. Some types can grow into thick sheets and some stay super thin and fragile. It's basically half bacteria and half algae. Doesn't matter what type it is exactly. They can all be treated the same way. Reduce over feeding to a bare minimum. The fish will look hungry but shouldn't lose any weight. So don't worry about starving. Just a piece of food no bigger than their eye ball daily. Literally no more than a single bite. This is going to be tough one because we all feel guilty about depriving our fish of food and enjoy seeing them feed. Next make sure your water source is not the cause. Have it tested and post here: ammonia, nitrite, nitrate, pH, phosphate, and anything else your LFS can test for you. If the tap water is less than ideal try using bottles jugs of spring water till things clear up. Keep vacuuming as much waste from the tank as possible. Excess amounts of fish waste can feed this algae. We want to starve out the bad bacteria/algae. The good bacteria will be fine so don't worry about that. Rinse the filters weekly but don't replace them. You want to remove any solid waste before it breaks down but keep the good bacteria alive on the filters. Next biggest cause is too much light. Reduce the lights to 6 hours each day and don't allow them to stay on past dark. The total photo period includes ambient light from the windows and you don't want to extend it. The plants won't grow as much but at least they won't die and melt either. The algae on them is probably doing more harm in stunting their growth at the moment. Each week you should see a huge improvement compared to the last.
Total black out of all light will give you the fastest results but some plants might not like that if you doing for too long. Jungle vals and anacharis can be funny about that.
Thanks for the reply. I suspect cyanobacteria, though it differs from what I had. The water parameters are fine. I test the water once a week and use osmosis water to which I add minerals. Fish are fed 2 days out of 3. Change water 25 percent once a week and clean the substrate. The aquarium is planted 95-96%, and I have 2 filters with pre-filters that I clean once every 2 weeks. All these are such a year and there were no problems. I suspect I brought the problem from another aquarium, it may have been more inattentive because I had a very busy time and I kept maintenance to a minimum. I currently use Esha Protalon 707 and expect results, combined with a thorough cleaning of the plants and UV on for 8 hours at night. So far the water has cleared and less cyano has appeared on the plants. I will try erythromycin if I fail to do so.
*** I'm sorry for my english!
I'm not comfortable with using biocides to combat algae. I've seen too many tanks crash after its use. Especially heart breaking when it's a reef tank full of healthy corals. I recommend to my clients to instead dose beneficial bacteria such as Prodibio Bio Digest. Not sure if it works on freshwater tanks but there are strains developed specifically for fresh. Works best if you dose a little every couple of days, instead of adding a lot each week. I didn't think it would work at first but was shocked by the results after the first tank I tested it on. Got rid of all the cyanobacteria and bubble algae in less than a month.