OK, so you have an micro algae outbreak... welcome to the club !
You didn't say where your water is coming from when you do changes. Do an on line search for your area's water quality if you are a city dweller. If you have ground water thats a different subject.
Tap water has pollutants in it including phosphate... fertilizer.
Are your lights on too long ? Is your lighting, whatever it be, old ? Lights dim after months and months of use which adds to the outbreak.
Do you have macro algae ? enough to hog up nutrients the micro algae is feeding on ?
Are you over feeding your fish ? feed the fish... not the tank !
Add the proper amount of snails and algae eating hermits. In sever cases, get yourself a sea hare. But most do not live long in captivity. A Sally Lightfoot crab is another great algae eater. Just see to it that the sea hare has plenty of algae to consume since thats all they live on... they stave unless you can provide them all they need to sustain life... you might even have to buy hair algae for them ! ( believe it, or not )
I's stay away from Lettuce Nudibranchs... they dont live long and they usually find their way into an impeller. The hairy legged variety of green crabs are good algae eaters... the hairy lagged variety eat bubble algae as well.