Generally micro means too small to see with your eyes. Very small 'micro-organisums'. It comes from anywhere, hitchhikes with live sand, rocks, corals, fish, from dead stuff when starting a cycle ect. Once some is there then may grow and reproduce to where theres enought together to see like algeas growing on the glass. Some will grow to be big enough that a single animal is big enough to see and then its called Macro organisums.