Hi..

This stuff is slimy and covers the leaves and back of
the tank. It's

Sounds like "cyanobacteria"..? These nutrient-hungry folks
are used to exist in so called "nutrient-poor" water.

How do you feed your tank? How many fishes do you keep and
how do you feed your plants..?

Finally: when did you start this tank..?
--
cu
Marco