I agree with the last post; also clean the underside of the cover thoroughly with a cleaner safe for vinyl...look around for one that says it will kill pseudomonas, since it sounds like you may have pseudomonas aruginosa in your tub. If it has advanced far enough, you'll actually need 100 ppm chlorine to kill it, along with a good scrub of tub surfaces, water change, and a filter soak for several hours in an even stronger solution (or replacement of filter). This is because it coats itself with a slime layer. Good luck!
Steve
P.S. I've been dealing with this just this week...perform a search of recent post on pseudomonas