Now if overhanging branches are creating the shade then it is possible that you might be able to easily take preventative steps cutting back the branches. Once it is established though, what can you do? Common forms of bleach or products like Jeyes fluid can be very effective when combined with good old fashioned elbow grease. You will also be able to find a raft of algae removers online or in your local DIY store.
Make sure to follow guidelines on the packaging such as protecting bare skin or your eyes and take care when using them in close proximity of plants that you want to keep. Remember to avoid using pressure washers if you can whilst this will clear the problem faster than other methods you could end up damaging your surface.