What makes algae?

[luckycharm]

I've got a 20Gallon tank that's having some problems with algae. I've got crabs and snails, a skimmer, I do water changes, I've tested the water and everything comes back with acceptable numbers. Its not hair algae..its not bubble algae. Its just plain gross algae that is turning the rocks green.
I also would like to add that I had some corals in my tank. I had mushrooms, some xenia, some zoos and some polyps that had to be removed. They were loving it but, without warning, they all started to shrivel up. Think there is a connection between that and the algae growth?
If all my water tests are coming back fine, what could be the cause?

 

[paintballer768]

Algae just happens, Im not sure how it really gets in there, LR I guess. Huge algae blooms are sometimes caused by phosphates. I think that algae uses up the oxygen in the water so they may have been suffocating. I know theres additives for algae removal, maybe more snails would help? Good luck