If it is organic, you probably have biofilm. Biofilm is when bacteria, sometimes even different types of bacteria, get together and form a protective coating.
You can tell if it is organic by getting some pieces of it in a jar and adding chlorine. If it desintegrates, it is organic. If it stays, it is plastic/part of the filter.
In order to break up the biofilm, you may need to superchlorinate the spa. This means getting the ppm of chlorine up to 100. This can be achieved by using bleach.
If you end up superchlorinating, do not leave the water in the spa for more than 1 hour .