I have run big Yachts for years
Probably in the same way magnetic "filters" work in ponds. The idea is that they alter the water's chemistry which prevents the algae growing. The algae "live" in the interface between fuel and water that almost always exists in the bottom of tanks.
Personally I think all these gimmicks are similiar to snake oil.
They may have some marginal benefit but I am dubious the claims made would stand up to proper scientific scrutiny.