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The biocover or biofilm to which you refer is composed of bacteria, diatoms, algae and little animals that form a layer on any surface submerged in water. These films are very variable, meaning that biofilms from different objects can have vastly different compositions of these organisms. For example two different rocks that are next to each other in a stream will likely have completely different organisms that create the biofilms growing on them due to slight differences such as water flow, light/dark, what colonises the rocks first, ect ect. My point is that given this variability, there is no way to get an accurate idea of the nutritional values of these biofilms. Also the shrimp may not eat all of the biofilms but only eat certain bits e.g. the algae and some bacteria, avoiding the diatoms. You can get around this by growing your own biofilms, e.g. growing algae on rocks.