Hair algae in a small quantity is, I think, kinda pretty. Unfortunately it is an aggressive grower and needs to be kept in check.
I'm not so sure what your tank is set up with, but if you have live plants its best to get more higher level plants in there to out compete the algae for nutrients. If you don't have live plants in there, its a matter of keeping nutrients that the hair algae thrive on to a minimum. Using algae killers is a temporary roller coaster of frustration. Sure it will kill the algae, or most of it, but once the algae die and decompose they release nutrients for another round of algae. You would need to continue using algaecides and do plenty of water changes, as well as staying on top of the nutrient load, which is a good idea anyway for the health of the tank and its inhabitants.
Algae can be introduced into any aquatic environment through a variety of sources. It can be introduced on plants, rocks, decorations, snails or even in the water that new fish arrive in. Algae is also air borne. Some species of algae spores are extremely small and float on the breeze. That being said, I'm not sure if hair algae is capable of being an air borne one or not. My personal experience is introduction on something, like cool rocks I find outside that may have hundreds of years ago been in water. Algae spores are tough and can survive a long time dried up. Its no wonder algae is so prolific. Amazing actually.
If you can put it in the back of your mind that when you see algae it means something is out of balance and the algae is attempting to correct it by consuming those excess nutrients. Algae is also common in any artificial aquatic environment that is still stabilizing (in other words, new). Its one of the reasons so many people leave the aquarium hobby in their first year. If they only stuck with it and had been given the knowledge to understand the dynamics of what's going on.
Algae in and of itself isn't a bad thing. Its natural. Anything in over abundance isn't a good thing. There's a lot of natural food for aquarium inhabitants to nibble on that live in algae. Algae is also handy if you have newly hatched fry. In my opinion, an aquatic environment without even a small amount of algae is sterile and unhealthy. It would be like if people had to live in a hospital and not get to go outside or get to eat ice cream or other treats. Its all a matter of balance.
Anyway, sorry. Bad habit of over answering. I'm glad to hear you guys are enjoying your watery worlds!