the removal of background coupled with the lighter surroundings as compared to the high tech planted tank surely is what the difference is. no background=more light at differing angles and the lighter color would predispose the fish in question to "adapt" its color to blend better in the surroundings.
an interesting experiment would be to move that fish permanently (or at least long temporary) into the other tank and move a fish from the other tank into this tank. wait a few weeks and see for color changes.
just keep removing the dead plant material as it falls off. encrusting lichens literally anchor themselves to rock(as well as some mosses) with microscopic hyphae, which is why it is almost impossible to remove it by scrubbing/boling. itll die eventually in permanently submerged environment.
cheers
