What is the slime at the bottom of aquarium tank?

It is exactly what you called it. "Slime". You should clean the filter media before this happens. Most successfull keepers of animals (pets) attend to the cleanliness of their charges/pets quarters and conditions very regularly and meticulously. My advice is to strip down and scrub the filter clean, and replace most of the media. Reserve some of the old media keep it wet and rinse it gently. This rinsed "old" media will innocculate the new clean media with the necessary bacteria to cycle the filter. Then service the filter regularly, keep an eye on the water quality in future and replace at least 50% of the water every week.