Those are rocks, and artificial plants. I probably do overfeed. So you only feed once every 3 days?It could be a number a factors like for example location if you live in an area where there are lots of insects like if you live near the Great Dismal Swamp. Or terrestrial potted plants near the aquarium due to rotting pot soil or even that you don't use screens on your windows.
The most likely cause however is that IF like most people your tank is overcrowded and you are overfeeding your fish. make sure you are honestly follow the 1 inch per gallon fish rule OR that you are feeding your fish no more then when they can eat in 15 minutes once every 3 days (my method), and I guarantee you you'll have a healthy looking tank and fish..From the photo of what appears to be a tank not overs-stocked. overfeeding is probubly the cause unless the drift wood you placed in the tank is too soft and the wrong species if you found it and is rotting all the way through fast over-reproducing bacteria bloom.
I don't believe that its over crowded. If it is, I don't think it is by much at all. like i said, i've got about 17 fish i think. most are maybe 3-4" long, some are less, and theres 2 that are maybe 4-5" long, so might be a little over the one inch per gallon (55gal tank)
I'm going to clean the algae on the underside of the lids, and change 75% of the water today or tomorow, and start feeding them less, and keep changing the water until i can get it clean again. hopefully itll clean up in time!