I don't even try to keep live plants with my big panaques anymore. If the green is fading, it's probably due to a deficiency in iron. Get a bottle of plant food, with iron, and your greenery should recover. As far as your royal dining on plants, that's highly probable. The only plants my young panaques seem to ignore are my dwarf and standard FW onion plants. Everything else is considered to be food by them. You can cut down their plant grazing a bit by offering them other fresh foods. I have some that like kiwis, others that like pumpkin. Most of my panaques will eat whole potatoes. A potato will last a couple of days at the bottom of the tank before enough bacteria builds up inside them to cause them to float.
The amount of driftwood panaques consume is based on the type of wood and the preference of the panaque. I have an 18" royal that grinds down 30 to 50lbs of driftwood a year. My other big panaques tear up only 10 to 20 pounds a year.