Is it actually slimy textured? Sounds like cyano bacteria to me. Pretty common. It likely has an unpleasant green, earthy smell as well. Manual removal will help, treating with an antibiotic will kill it. If it is just in the filter, remove the filter from the tank and clean it completely out. Disturbing it into the tank will cause it to spread--probably into low light areas.
While not pleasant, it really isn't a bad problem in small doses. I have a large colony that grows in the substrate. It never grows anywhere else in that tank, though, so I just leave it alone. In another, lower light tank, it will form a scum on top of the water in the floating plants. I sweep it off and am seriously considering using the antibiotic treatment--have to remove the newts first--because when it has access to air it really stinks.