We received a call a couple weeks ago from a pond owner in Texas that had a rusty tint to his pond. After seeing the pictures we identified his rusty colored film as a type of red algae. This algae usually starts out as green and then changes to red when it blooms. Once it blooms there is really nothing you can do to treat the pond. The key is prevention! Regularly treating your pond with pond dye and beneficial pond bacteria can help slow down growth. Once you get algae you will want to treat with a copper sulfate product like Crystal Plex.
Contact Sanco for more information on pond maintenance.
Texas Pond owner with red pond algae