Seems to be various answers when asked what causes it - too few phosphates, too many phosphates being 2 of them :-? Tanks with a high pH are prone to it (I have a full blown case of it in my Malawi tank at the moment but since there are no plants in there I just leave it. Fish won't eat it.
Remove as much of it as possible by hand then turn the lights off and cover the tank with a blanket for a few days. Keep an eye on the plants though as they will not cope without light for long.
Little water changes often may help as well. Do you do regular water changes?
I found nothing worked for me (I got an infestation in my tropical community planted tank from some plants I added to the tank) and finally resorted to using Erythromycin, 200mg per 10 gallons (40L?). This is a prescription only medicine so you need to sweet talk your GP. I work for a doctor and he took great delight in going up to the A&E dept at the hospital and asking for a few Erthromycin pills as his receptionist has algae! The Erythro will kill it off very quickly so it is important to remove the dead stuff ASAP or it will pollute the tank. Keep track of your ammonia and nitrite levels as it kills off bacteria - including those in your filters. I dosed my tank with an AquaClear 500 running to no ill effect that I could see but my tank was also very lightly stocked at the time. Be prepared to do water changes to reduce the ammonia and nitrite levels if required.