This can be caused by a number of things. Here are some factors which contribute to this:

- Algaecides
- High Total Dissolved Solids
- Soap Residue in Swimmers Clothing
- Improper Water Balance

TREATMENT

1.) Test all water factors and balance water as necessary. 
2.) If using algaecides, switch to a non-foaming type.
3.) Spas should be drained periodically based on bather load.