I've just found what I think is hair algae on my plants. Is it harmful and if so how do I get rid of it?

Any help is appreciated! Cheers!

Not harmful, just unsightly if in abundance.  To control try either dimming the lights a bit (if they can be dimmed) or shorten the time they are on by a couple of hours per day.  Also get a phosphate test kit and see if the levels are high.

Your plants will out compete algae for food and light but if there is any excess that the plants don't use then the algae will grab it and grow.  An aquarium is all about balance and too much light &/or nutrients will feed the algae, too little and there is not enough for the plants to grow.  Finding that balance can be just a matter of trial and error. 

I also take it you haven't done a water change since you are still cycling?  If so then the phosphate level will be high from all the rotting food over the last couple of weeks and a few simple partial water changes over a period of a week maybe all it needs. Bacteria doesn't get rid of phosphate like they do ammonia and algae loves it so will thrive on any excess the plants don't use.

Perfect! That explains it, and no I haven't changed the water but I will as soon as it cycles.