Well, I'm seeing why there is green water.
The fish will have problems if something isn't done relatively soon, I only mention that in passing. You mentioning the Betta killing the other fish made me smile; how often i write that Betta are not community fish, and i get told i don't know what I'm saying, they get along fine, blah, blah...well, they don't as you have seen.
Feeding once a day is sufficient; the more food in, the more waste organics, so cutting that in half will help the green water clear. Then there is light, it is too intense and too long duration. If the sun is on the tank that is doubly bad both for this and the temperature. But even bright ambient light in the room can impact algae. I discovered last summer that the reason i had been experiencing algae increases every summer was due to the increased daylight in summer, brighter and longer, even with venetian blinds fully closed. I added heavy drapes over them, end of algae.
The tank light has to be regulated according to the needs of the plants. If the plants can utilize the light, fine; but once nutrients are no longer available, the light remaining on is only going to feed algae, be it green water, brush algae, etc.
Hope this is of some help. To summarize, reduce the light and the feeding. Partial water changes more often will help to clean things up if the light and food are curtailed. When back to normal, once weekly water changes of half the tank, and then resolve the fish issues.