Joe, check into the Power Service products “Bio-Kleen” and “Clear Diesel”. Power Service is a good company with great customer service and advice, and I’ve used their products for years with no fuel problems. You’d use the Bio-Kleen first to kill existing algae, then the Clear Diesel to help disperse and remove it. There are other brands of similar products available. But a fuel polish like Gerald speaks to may be necessary regardless, if the situation is very bad.
Algae forms on water in the tank. The water usually comes from the coach being parked for long periods with the tank partially full, especially in humid areas or where temperatures have changed a lot daily. The water condenses out on the tank walls when ambient temperatures cool them, then the condensate goes to the tank bottom where algae can begin and thrive. The best preventative is to always store the rig with a full fuel tank, so moist air can’t occupy any room in it.
Joel