This might be wet wood or slime flux, and can be fairly common on trees that are not necessarily suffering ill effects from the condition. However, it could be overlapping with other factors contributing to decline of the canopy, and the tree should be evaluated by a certified arborist or licensed tree expert. Not all conditions (including wet wood / slime flux) can be treated or cured, especially if they have been present for months or years, but an arborist can help to narrow-down a diagnosis. They can also look for sources of tree stress that might be able to be alleviated. Aside from making sure it is monitored for watering needs (guidance on the linked page) during our ongoing drought, there probably isn't anything else you can do yourself in terms of curative or preventative treatment.

Miri