I'd take a pool skimmer (or a make shift one) to get as much as the algae as I could - then follow up with an algae abatement treatment
Would guess some invasive species of algae has made it's way into your pond.
The starting point is to check the ground wire for the battery itself. Often battery problems are the result of a poor ground whether the wire or the connection itself is shot.
A new ground wire is cheap, easy to replace and probably not a bad idea on a 16 year old vehicle.