Yep, sure looks like you have BGA in there, does your water smell musky? heck, If you touch the stuff does your hand smell icky too? 99% sure thats what it is just from the pics, and like Cheesehead said: :eek5: Did I just say that? lol.
It may be because of very low to no Nitrates at all in your water.
Its a good idea to keep your Nitrates in the 10-20ppm range at all times, its good for your plants, and it will prevent another outbreak if thats your cause for BGA. You can, and should start dosing with KNO3(potassium nitrate) or you can use "Nitrogen" from the flourish line(Seachem makes this)
What i've decided to do is to sort of start over. I had sand in the tank, but didn't like it as it got to messy to quick, so i'm changing my tank back to gravel. I have another tank, so my fish and the filter have gone into that tank, so as i don't have to cycle the tank!!!
Hopefully this time, which a little more knowledge under my belt i may be able to produce better results
You want to be careful not to transfer your problem into this new tank, if you didnt give that filter a good cleaning. Also, do you do weekly waterchanges, and substrate cleaning? A Dirty filter,and substrate are also causes of BGA, as well as stagnent water which Cheese mentioned.
Definatly start by testing your Nitrate levels, if these are low under 5ppm that may be your red flag for BGA.