Is that brush in the photo a stiff one or soft?
Try 1/2C laundry detergent + 1 cup bleach + 1 gallon water. Poor it slowly on, let it pool and sit for 10min. Any areas that dry out, rewet them. Then scrub hard with the (hopefully) stiff brush. If it's NOT a stiff brush, you'll need to get one and get down on your hands-and-knees and scrub it good.
Repeat, rinse & blow-dry.
Trisodium phosphate (TSP) + bleach will also work.....unless the Ca fruit-loops have banned TSP...lol.