What is this?

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.