Rest,
The basic concept is that the spa gets fresh water when your spillover is working. Without the spillover the spa water will quickly go south and you'll get algae. The Spillover needs to run an hour or two per day. Generally most owners just let it spillover any time the pump is running and they are not in the spa mode.
Do you normally leave the spillover off? You should be able to remove the "fountain" and just let the water enter the spa at the bottom of the spa where the fountain would normally attach.
You could also set your Return valve (the one on the far left) so that the spa always gets a little water, while the pool gets most of the water. But.. This is just about the same as turning on your fountain.
I assume you don't use the spa much?
The real problem is I suspect that you are not keeping your FC high enough to prevent algae.
My guess is that you are trying to keep your FC as low as possible, most likely in the 1 to 2 ppm range. (That said, I've been wrong about 100 time today alone, 1 more time won't make much difference..

)
Thanks,
Jim R.