Can someone please tell me what's this? Dirt? Algae? Something else? What can I do to remove this?

Sorry, I'm at work and in hurry, so I forgot photos:

http://i63.photobucket.com/albums/h141/iva...0002512x384.jpg

http://i63.photobucket.com/albums/h141/iva...0003512x384.jpg

It's definitely algae but not to worry, its not dangerous. Here is a good link I found for you to read.

http://www.kokosgoldfish.invisionzone.com/...showtopic=35167

! Now I know it's brown algae and I'm very confused about many different opinions how to treat this stuff - I've read here lots posts, and some people say it's happening commonly in new aquariums (like mine) and it will pass, the other are talking about algae eater fish or chemicals. What can I do? I can somehow clean stones, but which is the best way to clean plants??? Tank lights are on for 12 (sometimes 13 or 14, it depends when I come home) hours a day. Is this too much light? Should I use some products against this thing or just wait, clean and see?

I would definitely decrease the amount of light.
Maybe 8 hours? (until the tank has run for a little while longer)
My tank had an algae problem. I actualy left the light off for a couple of days and it went away.
A plecho (algae eater) might also be a good idea if you have room.

Good luck!

It should wipe off surfaces easy. If you are planning on an algae eater, do your research first because not all eat brown algae and not all are compatible with goldfish, assuming that's what is in this tank.

Don't use chemicals! Chemicals will kill your plants and make your fish sick. I don't even know if they would work because they only work on certain kinds. Brown algae isn't really algae at all...its a diatom so I don't think it would work.

Brown algae is caused by several things: low light, prolonged light, excess nitrates, and silicates in the tap water. First try rubbing it off, limiting light to 8-10 hours a day, and doing more waterchanges.

! I've rubbed it off, limited lights on 8 hours per day and done more frequent water changes; I think it's under control for now, but not completely gone.