Hi gt568,
Welcome to the forum! From what I can tell, you pretty much hit the nail on the head because it looks like hair algae. There are probably hundreds of individual species but this is a general category.
There are a variety of indirect causes all which lead to the direct cause which is poor nutrition. To get a better handle on the specific cause in this case could you give us an idea of your tank specifications such as size, lighting, substrate,water KH/GH? We need also to get an idea of what nutrients you are dosing, what quantities and with what frequency you are dosing. How long has the tank been running? From the looks of the growth and the amount of aerial roots it seems that it's not a brand new setup. Could you also tell us what kind of filtration you have and how often you change water and clean the filter? Is this a CO2 injected tank?
Hair/thread algae is a tough customer so it's better if you immediately remove any infected leaves. If you can answer the questions posed above we will have a better chance of diagnosing.
Cheers,