Something is running down my trunk — sap, or dark wet streaks — what should I do?

What is coming out tells you most of it. A wet, dark, slimy streak running down from a crack, an old pruning wound, or a branch crotch, drying gray or tan, sometimes smelling sour or fermented, with flies and wasps working it. White or tan foam bubbling out of the bark in hot weather. Clear sticky sap or amber blobs, especially with sawdust-like frass or a hole at the center. Dark rusty, oily-looking streaks running down from a sunken area of bark, where cutting the bark shows reddish-brown discolored tissue underneath. Note the location too: from the root flare, from a wound, or from apparently nowhere. Fluid leaving a trunk means internal pressure plus a path out, and what it means depends entirely on which of the above you have. Bacterial wetwood, or slime flux, is a chronic fermentation in the heartwood that builds gas pressure and pushes liquid out through any opening — unsightly, hard on the grass it runs onto, essentially untreatable, and carried by many trees for decades without decline. It is not a reason to remove a tree. Alcoholic or foamy flux is a seasonal yeast fermentation in stressed sapwood and usually passes. Sap with frass means an insect is inside. Dark bleeding from a sunken canker means an active pathogen is killing bark and cambium, and that one carries a real prognosis. Two of the four are essentially cosmetic; two are not.