dark crusty stuff?

[Bell]

I might be posting in the wrong section here.. but here goes. I have had a shrimp tank running for a couple of year now with cherry shrimps, a few ottos and pygmy cories. (I suspect the cories are eating baby shrimps but that's a diffrent issue) I have a recent problem with some strange crusty buildup coating the leaves of my anubias plants... at first looks like dark spotty algae and before I realise (OK after a couple of weeks) it it's become a dark crusty layer that will flake/chip off when i clean my tank. it's all over the place and turns a pale colour if i try to treat it as algae with liquid carbon... Can anyone explain what this might be and how to fix it?

[shinycard255]

Pic would be very helpful here...