Well, if you are indeed keeping them only under the 50 watts like you say, then you have to realize that YES they will fade. I've had shrooms that faded when placing them in different tanks with little light. They can go from whatever color they were before to an almost opaque same color, it’s strange, but cool to see how they try to adjust. I don't think that its dieing however, I think that it may still live fine a very long time, but it will just morph slightly to a different color as it's natural light requirements (as in nature, or the previous owner) are no longer met in your tank. So it won't look as pretty, but may still not necessarily die. I’ve had shrooms that change colors so dramatically under direct light than when taken away from direct light, or in very low light, it looks like a completely different specie!
Anyway, yes this is normal. AND, although many people say mushrooms have low-to medium light requirements, I’ve found that many (with proper acclimation) will LOVE high light and some drastically change colors to even prettier colors when under such high direct light. Acclimation is the key though, as I’ve also seemingly burnt some shrooms, but they too eventually regain themselves.
Take care.