You might want to consider changing the water. Sounds like it's gotten a bit yucky even tho it hasn't been used much. Usually spa water should be replaced at least every three months.
If you decide not the change the water at this time: you need a good test kit so that you can test the water yourself and not depend on the pool store. You only need to test for total bromine (unlike the chlorine tests), so a DPD or OTO kit is fine. I think the six-way kit available at Walmart will do the trick for now.
Get the test kit before you do anything else.
With the "5+" reading you reported, I suspect that 5 is the upper limit of the test used by the pool store. It may be a LOT higher.
A total chlorine reading can be converted to a total bromine reading by multiplying the TCx2.25. I think the OTO's maximum reading is 5 (for Cl), which would make it about 11 for bromine. If you're topping out on that test, try diluting your sample with distilled water, one-to-one, run the test again and double the reading.
TA should be around 100-120 in a bromine spa. Yours is a little low but not a big problem.
You don't need any CYA in a bromine environment, so don't worry about that.
I've never heard of a bromine generator, so can't help you much there.
If the bromine is excessively high, you'll need to let it drop some before adjusting the pH. High levels of chlorine or bromine will bias the pH test. But, if it still turns out the the pH is high, use dry acid (aka sodium bisulfate or pH Down/ Minus/ Decreaser) to lower it. pH is handled the same way whether you're using bromine or chlorine.
Since you're getting scum, it sounds like you need to shock. I shock with 10 oz of bleach (yes, shocking a bromine spa with chlorine is appropriate), run the jets to circulate it, shut them off and cover the spa overnight. Then in the morning I open the cover (for about 3 hours) and let the sun break down the excess bromine.
Test the bromine to make sure it's at an appropriate level (3-6). At this point I like to add an enzyme treatment. Enzyme products are especially helpful in a spa due to the high level of organic contaminants. Spa Perfect (Natural Chemistry) and Scumbuster (from Spa Depot) are examples of such products. The enzymes help out the sanitizer in removing contaminants and help prevent that scum line. A scumbug, scumball, or Zorbo is helpful too, especially in conjunction with the enzyme treatment. This is a sponge-type object that you float in the spa that absorbs the scummy stuff.
When you shock the spa, this destroys the enzymes, so fresh enzyme must be added after each shock, once the sanitizer level returns to normal. A spa should be shocked weekly; more often if there's a high bather load or the water is looking hazy.
