Algae

Well, I'm not doing too well at keeping this updated....I guess I haven't done much except work recently. Anywho...
Let's talk about algae, found on rocks at the beach, in an aquarium, on the stems of the plant in my fishbowl and surprisingly enough, in the bottles that contain my drinking water. It has happened now in both my brita and in my other bottle. I can get rid of it no problem, but my question is, how did it get there in the first place. Any ideas? At least it's better than mold.