Have not seen that described as a disease or insect damage that affects Dogwoods.
Possibly a soil borne fungus that has migrated onto the bark of the tree at the ground level.
If there is mulch on the soil, and it is touching the tree trunk, remove it from around the trunk and leave a gap of 3-4 inches to permit air circulation. Wash off the foreign matter and inspect the bark for damage or insect penetration.
If there is none, then there's nothing to be concerned about. Borers do attack Dogwoods, but their trailings contain wood pulp and have never heard of any white residue.
Any damage to the roots should be reflected in the foliage, such as curling, drying, premature coloring, leaf drop, etc.
Keep us posted.
Rb

It seems to be gone this morning. How odd! It had rained the night before, and I had it on two different dogwoods, but by afternoon, it was gone. Hmmm....