Whats up with the water in my bettas tank?

so i have 3 male bettas in a 10 gallon tank. and it is divided. now im slighly confused with whats going on with the water. on the left and right hand side the top of the water is clear, but in the middle section it has a greenish film over it.. what is that, and why is it only in the middle section and not in the others