Rabbits' water greenish color

My rabbits are in a pen on concrete and the watering device is a 2.5 gal gravity type. The water comes from my well which seems to produce good water. A day after the most recent refill, I noticed the water has a greenish tint. This doesn't look like algae as there is no scum or slim like the kind I see in my pond. It looks more like what you would see with a little food dye. Should I be concerned? Suggest a remedy?

Algae comes in different forms.
Easiest way to handle it is to clean out the waterer and scrub down the inside with a little salt and water or vinegar to make a paste. Rinse well and refill.

If you scrub it, then keep it out of the sun, it should stop the green. It will also keep the water cooler, which is better for your rabbits, as they are more likely to succumb to heat that cold.

I agree, sounds like algae.  And yes, keeping it out of the sun is huge for minimizing such issues as algae are a plant that uses photosynthesis to help make their food.  

Vinegar in their water should help to suppress the algae growth if you find it still is a problem after cleaning the waterers and moving them to shade.  Lots of people on permies swear by raw/unfiltered apple cider vinegar in their animal's water, so I'd go with that, rather than something like distilled vinegar.  I've not used vinegar in my animals water, so I'm not sure on the appropriate dosage, but I'm sure someone will be along to explain that shortly.  If not, the search function is bound to turn up a ton if information.

I've found that mine only turns green when I am not up to date on cleaning out the bottle, so instead of covering up the bottle to avoid sun I just try to keep up. I actually sized down the water bottle (I have one that uses a standard soda bottle) from 1L to 600ml for exactly this purpose.