Hi, 

It could be a type of slime flux on the bark which is mostly nothing to be concerned of. It could also be some sap oozing from a wood boring insect, or a combination of both.

You can consult with an arborist or licensed tree expert to have them look at the tree in person and make sure that otherwise the tree is healthy and not a safety risk. The links will allow you to search for one in your area.

If you can mitigate any stress, like keeping it watered in times of drought and applying an appropriate amount of mulch, that can help with any symptoms of decline. 

Emily