How to eliminate algae in waterbed?

I have been out of state and neglected to treat my old waterbed at my parents' home. There is now algae inside the mattress. I think it is beyond the point where the typical "Waterbed Treatment solution" will help. Is there anything I add to purify the water inside or is it a lost cause?

NO NO NO to the Bleach! Bleach is a corrosive and it may weaken or destroy the vinyl. You may end up with a leaky bed or worse. You want an algicide that is designed for waterbeds. It can be found either online or at any store that sells waterbeds. Follow the instructions.