It appears to me to be some kind of alternative sanitizer stick (like Nature2 or something similar). The reason I made that guess is 1) the threads that could screw into a filter and 2) it appears to be sitting on top of a spa cover. But then again, I would expect a sanitizer stick to have some holes to 'release' the sanitizer...so hopefully someone can drop by with a definitive answer.
Is there anything similar in your spa filter?
On a side note, I noticed 'TBD' in your sig box for filter and spa. Do you mean you are waiting to decide what to buy, or you have not identified what you have? If it's the latter, you are welcome to post some pics and we can help you figure out what model they are!