We think that this looks like a slime mold.  A common name for slime mold is dog vomit fungus. This fungi consumes bacteria and other organisms living in the mulch. They use mulch and low growing vegetation as support for their reproductive structures and do not cause disease. Slime molds are not harmful to landscape plants. Growth is usually halted with the onset of dry weather and the masses dry up. They can return with wet weather.

Try to lightly brush some of it off the trunk with a broom. If it is dark underneath the crust, it is okay and no need for control. Take a look at our web page on mulch. Scroll down to  mulch problems and look at the links on slime molds. https://extension.umd.edu/hgic/topics/mulch

If this is not what you are noticing, you can send us more photos. 

Marian