Originally Posted by wkndracer
If that green easily wipes off the glass with just a rub of your finger then it's GDA (green dust algae). My tanks have it pop up when I'm low on phosphate and high on nitrates. Suggest if it's not a huge PITA to clean the glass and do a large >50% WC right after. Change the bulb in the fixture. Some bulbs shift value faster than others, cheaper fill gas and the spectrum changes within a year while others seem to hold value for several years (save the old bulb in the tube supplied with the new one).
The algae doesn't come off that easily. I have to use quite a bit of "elbow grease" and scrubbing to get it to come off.