Grime on the back of my tank?

[Phoxly]

Okay maybe its just me but there is some odd grime (dark green/brown) on the back of my tank, clearly visible after adding my Co2 kit and one of the suction cups was drug up the wall so I can see the path it made... I've never scrubbed the inside of my tank but this is weird. Its like dust but on the inside. Could it be... ALGAE? >__<

It is kinda spotty, like a lepur, I also notice one of my plants with leaves on it is kinda grimy looking too.. I think its algae... >.< Maybe I should get that cute otto I saw at petco yesterday haha?

[Welsh]

Sounds like algae, I usually get it in my planted tanks and the CO2 is most likely encouraging the algae to grow more but i'm not really familiar with Co2. I don't know if your tank is already overcrowded but an oto or 2 would help maintain the aquarium, I have two that I bought this week and they have done a great job just make sure that you supply them with algae wafers and blanched zuchinni when the algae runs out