Healthy plants with leaves that turn brown. What is it?

I know I've started a thread like this before, but I cannot find the source of my problem with new leaves turning brown after a little while. The plants are healthy and put out nice new bright green growth all the time. There is no apparent "algae or beard moss" and I have two SAE's that keep the aquarium clean. I do weekly 50% water changes, feed sparingly, have 2 LED light strips and use Seachem flourish. I have hard water. Could it be iron or other minerals in the hard water?
Ammonia is 0, Nitrite 0, Nitrate .5, no chlorine, PH 7.5, Hardness 250. Temp is a steady 78 degrees.

A LFS guy told me it was "slime". I'm assuming it's a type of algae? It doesn't rub off the leaves though. I have anubias nana, anubias barteri, java fern, Criptocoryne, vals, fern, and anacharis. The Anacharis seem the least affected. But all the other plants have the same problem.

What is it and how can I fix it.
Thank you for your help in advance.

Perhaps it's "sludge"? I found a product from Dr.Tim's Aquatics called Waste-Away that breaks down this type of stuff. My question is, will it work on dissolving the brown stuff on my plants? If it isn't sludge that's turning my plants brown then I guess it could't hurt as long as it doesn't change the PH of my water. Any help, suggestions or recommendations would be helpful.

Oh, I forgot to mention in my first post, that I only leave the lights on in my tank about 6-7 hours a day.