I had a similar experience with my water feeders; while the water did not turn green, they started to 'grow' something on the inside.
I have had them 8 months or so with no growth and all of a sudden, growth on the inside. I wondered if my weekly cleaning with a sponge and just tap water had caused grooves giving the substance somewhere to take hold, so I purchased three new containers. Within a week, same growth; probably because we are just heading into our summer, warmer weather etc.
While I used to thoroughly wash them once a week with water and sponge, I have now added detergent to the mix and always give them a good rinse afterwards. I note, however, that towards the end of the week, they need doing again but they are nowhere near as bad as they were before the detergent was added to the process.
While I do not want to start a debate on the pros and cons of ACV, I use it and have read that it helps to keep the water containers clean.