algae will start growing in the water column before it attaches to the walls of the pool. If you are getting algae growth with a SWG then something is not operating or set properly or your Salt, pH or CYA are not at the proper level.
my 2 cents!
I can see the need for shocking if you have has a party and the pool has has a lot of use or if a kid has had an accident in the pool, in which case I would use bleach,but otherwise I firmly believe that a properly set up and maintained SWG pool will not need to be shocked!