The problem is not the lighting. There actually is no problem whatsoever. Sheet moss is sold in a dormant state. It needs to recover. It will green back up, it just needs time.

As far as the lighting goes, moss will do fine in intense light as well as diffused. It will just have different growth habits. 6500-6700k is optimum for vegetative growth. Uv light is not required and as a matter of fact, is wasted on a dart tank, due to the glass on top, sealing it. UVB is a short wave radiation that gets blocked by glass by about 95+%. UVA is a long wave radiation and only fares slightly better penetration through glass.

Here are examples of the same moss, from the same package, under different intensities to show the different growth habits I mentioned.
This first one is sheet moss grown under T8 6700k bulbs. Notice it is still brown, but growing very leggy
And the same moss, grown under T5HO lighting. It grows extremely compact and bright


Moss can be one of the most trying plants to grow. It requires a lot of patience. Even more so when converting aquatic mosses to a terrestrial environment. But that's a whole other topic lol


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