Step I would take to get rid of the slime and to be sure it doesn't recur.
Add 1/2 gal unscented bleach. Run all pumps on High and uncovered for and hour.
Dump the water.
Rinse of the filter and soak in a tub with another half gallon of bleach for a couple hours. Agitate it every 1/2 hour.
Scrub the tub and rinse. Refill.
Put filter back in
Repeat all of the above.
Balance water. Check it regularly.
Going forward, use either bleach or lithium hypochlorite weekly, at a minimum. Heavier usage will require more frequent additions.
If you are using bromine tabs, put three of them in a floater weekly and don't forget.
Even it there there are no visible issues, change the water regularly. With light residential use (2 people, 1 hour/day), I like to see it changed at least every 3 months. More usage, more frequent changes (4 people twice a day = every two weeks)
Soak the filters overnight in TSP at water changes.
Happy Holidays!
Scott