Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Green water algae often starts as a grey or white colored cloudiness before you can really tell that it is algae. After a few days the green gets more intense, and you can see, as in your pictures, that it really is green water.
Another way to tell is to put some new white filter media in the filter, such as polyester floss. When it traps the green algae cells the green shows up pretty well against the white media.