You probably don't have a prayer because once infected mold will continue to grow until the seats are replaced.
Clean it with Spray Nine, and then bleach it with Chlorox Cleanup, and then seal it with Lemon Pledge or Armor All. (Yeah, yeah, someone will say that's dumb but you should see the upholstery on my 35 year old boat)
Actually, in recent years I switched to 3m's Mguires seat protector on top. Its still makes it shine but not as sticky.
Truly your only hope is to get the boat into indoor storage, and then do all the above, and then get the entire boat and bilge perfectly 100% dry. Then cover the boat with a simple bedsheet to keep the dust and spiders out. Then be sure to dry it out after each use.
If storage isn't an option, plan on going thru the whole rigmarole every month or so.