I went and examined things more closely. The rusty stub is actually a shrader valve - like a tire valve. And I can just barely make out the words pipe plug on it. They musta pressure checked lines when it was redone roughly 2 years ago and forgot to remove things.
I have a booster pump - an old purex with a bronze housing - that goes to the spa. It is not connected to the timer. Periodically, I turn it on for a few minutes just to flush out the lines and get some chlorinated water in there. I have noticed that the spa gets a little discolored when I do it, but it quickly fades.
This morning, it did it again, so I shut it off and decided to check the basket for organic debris. Hey, I've only owned the place since October, didn't want to rush... What I found in the basket was a black rubber float of some type, rather soft and gooey, like a toilet flapper that needs replacing, and a chunk of rust that used to be a chain of some sort. The float is about 1½" diameter and 3" long, with a steel shaft stub out the end. (also rusty)
Can anyone think of a reason there would be a float in the booster pump?
It was too big to get through the plumbing. It had to be put in there. Some sort of check valve? Needless to say, it's not in there anymore, nor is the rusty chain, nor is the basket at the moment. I'm debating whether I want to try an acid soak to remove the embedded rust or go look for a replacement basket.