Light encourages algae, and in my experience residual sodium sulphite tends to encourage growths of various sorts. Drain it, or make sure it is flushed out (there should not be residual chemicals after a completed wash, but who knows?) and kept dark. With mine, I close the siphon, run the water until the siphon starts, close off the feed, and leave it to drain. Then I know I am starting with fresh water. I also cover my washer with a lid when it is not in use.
It's a pain to have to get algal slime out of any container.