My plants look like they need to be dusted...

Hmm, all of the LFS's from around here carry Mondo Grass and display it submerged, that sucks, we'll see how long they live. Anyway, yea the stuff comes off if you brush it.
 
On the val, it could be a bit of diatom algae--the blooms usually wear themselves out over time. For the mondo--it will survive submerged for a long time, but the leaves ultimately need to be above the surface of the water, or it will die. If you can pot it and have the roots under water but the leaves above, it will live nicely.
 
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