Hi, 

It looks like it may be a type of slime mold. You can monitor and see what happens as there isn't any treatment you can do for it. If it moves/travels along the trunk or dries up with in a few days, it is probably a slime mold which most likely will be harmless to the tree. It could travel to other trees or areas. Slime molds aren't true fungi but similarly send out spores to reproduce to find bacteria or fungi to eat. Sometimes this works in a decay process on dead or dying material. 

There are many different types of slime molds and they appear in different colors and and shapes. Here is one that can be common on turf grass: https://extension.umd.edu/resource/slime-mold-lawns/

Let us know if it acts differently and you have further questions. 

Emily