Blue green algae (Cyanobacter bacteria). It comes in a range of colours from various shades of green, blue, black, brown, red, purple and pink.
It's a photosynthetic bacteria (bacteria that can photosynthesise) that loves red light, nutrients, low oxygen levels and dry fish food. It is common in some tanks and never seen in other tanks. In newly set up aquariums it can be a problem while the tank stabilises.
Reduce the amount of dry food put in the tank.
Make sure there is plenty of water movement.
Do big daily water changes and gravel clean the sand every day for a couple of weeks to physically remove it form the tank.
Have some floating plants in the tank.
