Without a photo I can't say for sure but it sounds like bacteria. The slime is a bacterial flocculant and the rust colored growth is likely cyanobacteria or diatoms. I would have to see to know which but likely the former.
How old is the tank? Many tanks, especially marine tanks, go through a period I call, "the uglies" which occurs shortly after the cycle and normally goes away on its own with regular water changes. In an older tank however, this can mean a problem with nutrients like nitrate and to a lesser degree phosphates.
An overgrowth of cyano can cause problems as it dies off it reduces the oxygen levels in the tank. Because of that I recommend that when you do water changes you also siphon out the cyano (if that's what it is).