Is there any clue in the fact that, two days ago I put in a large chunk of GAC in my aquaclear PH 'quickfilter' cartridge and, without any shadow of doubt, the water went from cloudy white to cloudy VERY green???
Depending on how long the carbon had needed changing...yes. The new carbon would have removed any yellowing agents very quickly. The nutrients in the system are undoubtably high, by removing the yellowing agent the amount of light increased rather quickly and boom, algae bloom.