Hair algae is closely associated with poor CO2, however, "poor CO2" comes in various flavors:
If the CO2 content in the water were zero and the KH were zero (or if the plant were unable to use CO3/HCO3) then this magnitude of "poor CO2" would be fatal. This almost never happens though because as long as there is CO2 in the atmosphere then a tank exposed to the atmosphere will have the CO2 dissolve from the atmosphere into the water at some equilibrium value, simply due to the partial pressure of the gas.
The other flavors of "poor CO2" are always within the context of how much light the leaf is being bombarded with. Photon energy collision with chlorophyll in the leaf starts a chain chemical reaction which requires CO2. The more energy received by the leaf the more CO2 is required by that leaf. The most frequent of this type is:
a) high light and poor injection.
b) high light and poor flow/distribution which fails to deliver the CO2 to the leaf.
c) combination of a) and b)
The final flavor is where the injection levels may be OK but the concentration levels are unstable. This is see in the following cases:
d) CO2 is not turned on early enough before lights on enough to saturate the tank. The level is too low at lights on but may rise further into the photoperiod.
e) injection rate varies during the day. pH controllers can be guilty of this, but constantly fiddling with the needle valve can also be a culprit.
f) low light, non-injected planted tanks when water changes via tap are performed.
Any combination of the above will undermine CO2 uptake and is exacerbated by the light energy. Lowering the light should always be the first step because light causes a demand for CO2, CO2 causes a demand for nutrients.
Further details can be found in these threads:
co2 and water changes
Stable CO2 - What does it mean exactly?
Cheers,