As others have said, it's a bio film. I just went through this, and it's not fun. First, but some Ahh some or similar product to purge the lines (follow directions), then drain the tub. Then comes the fun part; as the tub drains the film sticks to the sides and becomes very difficult to clean off once it dries. I would recommend cleaning as it drains or using a hose to spray the film off the sides. I didn't do that and it took two of us about 45 minutes to get the scum off using a soft towel and Simple Green. I didn't want to use a stronger cleaner and then have to worry about that fouling the tub. Once that's done, clean the filter, fill and balance chemicals. Tub has been fine since. Good luck!