what is this on the surface of my tank?

[tt101]

hey guys,
what the hell keeps appearing on the surface of my tank? its like slimy and it forms a film or almost a skin and it keeps coming back everytime i clean it off. im trying to post pics but it wont let me. its like these little black or dark brown pieces on the top of the tank. there are some pieces stuck to my live rock too. sometimes when i put food in the tank they dont go to the fish because that film is on top of the water.