Mulberry leaves are very high in protein, 21% of dry weight, and contain some minerals like calcium and potassium in non-negligible quantities (yay for sheep fodder studies!) Spinach doesn't have nearly as much protein but beats mulberry leaves in virtually any other category of nutrition.
Like anything, feeding one or the other of them exclusively is likely to be less effective than mixing. If you're not wanting to buy mulberry leaves online, just go pick some maple or oak leaves, rinse them off, and toss them in. You'll probably need to weight them down a bit, even if you try parboiling them, but once they're waterlogged and start to decay a bit the shrimp will happily pick them to bones in short order.
