Is that from the surface? It looks like the white fungus that naturally occurs when mulch decomposes. If so, it should not have any effect on the rosenka. Is the plant getting enough sun, they hate being in the shade, and do not like wet feet so should not be watered too regularly.

Fungus thrives in a moist environment, so it could be a sign that you are watering too much. Also, mulch should be several inches away from the trunk of the plant.