ClO2 for spa disinfectant and algae

I have been thinking about switching to ClO2 for spa disinfection. Our spa started turning slightly green recently so I put in 50 grams for a 250 gallon spa per the spa guidelines. I was shocked when the spa turned bright green! Since there is visible algae, is that dosage sufficient. I repeated the dose and it did the same thing. Any feedback would be appreciated.