Algae and other pond gunk has a habit of hanging out where you least want it, but take a deep breath before you reach for the power washer.
A small amount of algae is normal for a healthy pond (remember, it's a living ecosystem, not a swimming pool.) Only Americans, in fact, really seem obsessed with complete algae eradication, while many European pond enthusiasts relish their relish-colored water.
Washing your rocks will remove pieces of the ecosystem that your pond has spent so long building up. This ecosystem is so complicated that we can never replicate it with chemicals, so we never want to scrub or pressure wash rocks or anything else in the pond to the point where we get rid of it entirely. It will come right back anyway.
If you must give your rocks a bath, remove debris and gunk as gently as possible and rinse them with water from the pond instead of a hose.