I dont know how old your spa is or what brand, but some of the old spas used black air hose for a few years. It would break down over time and spit out black particles that kind of looked like mold into the spa. It was actually the air hose plumbing.
Some companies actually sell replacement air jets that are solid. They also sell plugs that can be glued into the jets. I myself like the air blower, but if you have an old full foamed spa that has to be re-plumbed from the airlines breaking down, it is an option.
Even the old spas that I have worked on turn the blower on at least once a day. Usually for just a minute at the begining of a cycle. Does your spa have adjustable filter cycles, or does it just filter when heat is required.
I don't know how old or what brand my spa is either. It was here when we moved in. It was definitely mold though. It was quite obvious. This tub does not have adjustable filter cycles. Actually, I should say if it does, I'm not aware of what to look for.
Thanks again to everyone for their help. We went into a slight panic at first, but we're over it. We'll just start over again when we're ready and know a little more now. Over the last two years that we've lived here, we seem to learn more with each refill and each visit to our dealer and this board.