It looks a lot like blue green algae.
If it lifts off in a film/ sheet and smells musty/ mouldy, then it is blue green algae (Cyanobacter bacteria).
You can take the rocks outside and hose them off.
Reduce the dry food going in the tank.
Increase water movement and oxygen, especially around the bottom where it is growing.
If you have a lot of red light on the tank, add some blue light.
If you have fluorescent globes that are more than 12 months old, replace them with new globes that have a 6500K rating. (K is for Kelvin). You should replace the fluoro starters when you replace the globes.