Blue green algae (Cyanobacter bacteria). This is a photosynthetic bacteria that loves nutrients, low oxygen levels, minimal water movement, and red light. It loves dry food that sinks to the bottom of the tank.
It does release toxins and is poisonous to most fish, shrimp, snails and plants if they are exposed to large amounts. However, a little bit will not harm the fish or other organisms.
It can also be harmful to people and terrestrial animals so don't ingest or inhale it, don't touch your face (mouth, nose, eyes) after handling the stuff.
Always wash your hands with soapy water after handling blue green algae or working in an aquarium.
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HOW TO DEAL WITH IT
Reduce the amount of dry food going into the tank. Use frozen or live foods instead.
Do a 75% water change and gravel clean the substrate every day for a couple of weeks. Try to suck the gunk and bacteria out.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Increase water movement and aeration in the tank, especially around the bottom.
If you have old (more than 12 months) fluorescent lights above the tank, replace the globes and fluorescent starters.
If you have LED lights, increase the blue and white light and try to have a light unit with a 5600K (K is for Kelvin) rating.