Hi robbo,
this could be what they call black algae or hair algae. this occurs if you have too much nitrate present in your tank or if your phosphate level is too high. You can rule one or both of these problems out if you test your water with the relevant test kits available.
Another reason could be that you may have your lights on too long, I have mine on for 10 hrs a day, can go upto 12hrs, if your tank is in direct sunlight or gets the sun during the day, I would stick with just 10hrs.

Some algae in the tank is a good thing tho, it shows that things are in good health and balance in your tank. But if it gets unsightly and out of hand then all is not well, and if you dont sort it quickly it can have adverse affects on your plants and fish.

I would suggest its better to try and get the balqance right, rather than bunging in lots of chemicals, which I know they say dont harm fish, but the natural way is best, and cheaper too. and its best to find the reason why, rather than keep bunging in stuff to solve it time and time again.

Please bear in mind I`m no expert, and I have the same fluffy hairs on my plants, and would appreciate some advice myself from ones that know!!!
But I hope what I`ve suggested is a good place to start.