"blanket" algae problem

My tank which has been established with no changes in care, plants, filter,
cleaning for 5 years has all of a sudden got an algae problem. It is a dark
green algae that blankets itself across the stones. Nothing on the plants
or glass. I've done two 20% water changes within the past week. I changed
the filter product, but not too closely to the water changes. I have spot
cleaned the tank of all algae, and overnight the algae comes back. What
type of algae is it? What could be the reason for it when I have changed
nothing at all? And waht should I try to fix this problem? I do have a
water test kit, and I will do a test on whatever one thinks the problem
might be.