We often see the velvet-like growth of eriophyid mite galls on silver maple and river birch.  Every once in a while we’ll see similar symptoms on Viburnum. The growths can be red or tan in color and if you magnify the sample to about 40X, you may find the small worm-like mites that cause the plant to grow this way. Damage is purely cosmetic. Mites are very host specific and there is no need to control in most cases.