Certified GKH Gardening Expert
It is most likely sap and dogwoods tend to sap when they are stressed. What is causing it can be one of several things. It can be everything from the fact that the tree may be getting old (dogwoods typically only live 10-15 years) to a pest of some kind. I would check the tree carefully for pests, particularly in the junctions were branches meet. Also, make sure to give it some fertilizer, to help keep its health up.