Welcome to the Forum!

It's hard to say without more info, but it sounds like some sort of water mold. Is there any chance you can post your free chlorine level and either the combined or total chlorine?

I assume the tub spills over into the pool normally, how often do you isolate the tub (i.e. use it as a hot tub without it overflowing into the pool)?

What I suspect is that you have lines (pipes) that are rarely used and allow the whits stuff to grow in them which gets 'spit out' when you isolate the spa and run water through those lines. A shock dose of chlorine should kill it, but isolate the spa and only shock that - it might take a couple of days and repeated additions of chlorine. Please remember to run the pool, without the spa spilling over for at least a few hours a day - you don't want to fix the spa problem only to have an algae filled pool!

I've done enough speculating for now, please get back with us if you have any questions and to report your progress.