Why does the prebiotic Multi-Layer Hyaluron Cream smell like mushrooms?

This is because the cream does not contain any fragrance as they adversely impact the microbiotum of the skin.  
You must have a good olfactory sense to detect that indeed the cream contains Schizophyllan which is a polysaccharide (prebiotic) from mushroom Schizophyllum.  We could add a frangrance to mask the smell of Schizophyllan but it works contra productive for the microbiome and results will fade away.