If connected to a pool, the best preventative measure will be maintaining the proper PH and chlorine levels of your pool. Using an algaecide in addition to treating the pool with shock before adding a phoshate remover, etc., and all other steps involved in opening a pool will prevent algae growth, but also using a water softener when the water hardness is too high and closely monitoring CYA levels will prevent any staining. For removing visible algae on the water feature itself, I'd try to use a stiff brush before moving on to more risky methods like pressure washing or cleaning with a bleach and water solution as this could affect water chemistry.
Outside of algae growth, the cleaning steps will differ depending on metal type. For copper, TarnX could be used to restore the original appearance but no other cleaning is recommended as the patina will darken over time. For 316 stainless steel, a scouring pad is recommended.