Green pond water: newly created ponds
Pond water is never completely clear. However, when it becomes so discoloured and green-soup-like that visibility is restricted to no more than a few centimetres, you most likely have an algae plague.
In the case of newly created ponds, it is far from unusual for the water to temporarily turn green. In the early days when the ecosystem is not yet fully established and properly aligned, suspended algae can quickly take over.
But why do suspended algae turn pond water green? Most microscopic algae photosynthesise. To do this, they contain green chlorophyll, which ultimately turns the pond water green.