I learned it the hard way when I tried to clean the algae (and other junk) behind the light. Basically (I was in the water),
1. I disassembled the light / housing (the cord was long enough I left the housing on deck)
2. Scrubbed (with a small brush) the inside
3. and all these "junk" (including dead algae which got "stuck" behind the housing) came out and started to spread all over the pool
One idea is to have someone place the vacuum hose (e.g., the end of your KK) close to your work area / hands so that you can vacuum directly into the filter. The junk will eventually get there through skimmers but as you know, it will take some time.
Oh..one additional thing I did (as I was fighting with rust and spent more money than I wanted on ascorbic acid)....since I had the light disassembled (and we weren't going to use the pool for a few days at least), I dunked rusted screws (not much but to be safe) in vinegar for a few days.