Here is the red (algae? , bacteria? ) stuff that grows only in the rainwater. I've seen stuff filled up with tap water and left outside only grow green algae. Its really weird and it grows wherever rainwater stagnates for at least a few weeks. I can't find any information on this, have asked people who are into biology and still nothing.

Red Algae is associated with the deadzones in the oceans though...



Here is a closeup