Hair algae, or commonly called green hair algae (GHA), is a type of algae that appears to grow in hair like appearance throughout the water. The algae is commonly found in a variety of setups regardless of the age (and in many cases is the first type of algae is be shown inside of a pond due to it's rapid growth).
Hair algae is a type of algae that appears to be like hair growing, and can be light green, greenish brown, to even dark green. This type of algae can grow in almost all areas of a setup, from substrate, decorations, glass, near any filter intakes and outputs, to even near the edges where the water meets the air. When this algae first starts to appear inside of the water column, it may appear to grow in a bush like manner, having a wide spread across where it is growing before it will start to grow hair like strands outward.
Hair algae will grow in any aquarium that has a balance between most water parameters, but may have either brought in an item into that contained hair algae, or having the lights on for too long. Normally when ones sees any hair algae growing inside of their water setup, it will be combined with thread algae as well. Hair algae can grow extremely rapidly depending on what the available nutrients are inside of the water. In many cases when trying to remove this algae it can grow back to its original size without hours if given the perfect environment to grow within.
Hair algae is mainly caused by the lack of having any competition to feed on excess nutrients, having water quality that isn't good (excess waste, nitrogen cycle isn't fully setup, or other aspects), having the lights on for too long, and even from placing items from another tank or unknown location into your water. By making sure that you inspect, clean, and check where you get all of your decorations, substrate, or even equipment before placing it into your water. By staying on top of water changes (including to make sure that you have a nitrogen cycle fully going), you can remove the issue where water parameters might be still swinging between different values.
In many cases adding live plants into the water can create some competition between who can use all of the available nutrients first. Thus, these plants will actually starve most types of algae from ever starting to grow or fully take over. Others also will use fish or inhabitants that eat this type of algae if they are unable to get a full grasp on how to fix the root causes, although this is not recommended since once the issue is corrected you will need to supplement the newly added inhabitants with a supply of algae to feast off of.