Had you done a lot of low speed running or trolling prior to coming into the ramp?
Chances are it is a mixture of water and oil dripping down the carbon coated exhaust pipe. If a thermostat or pressure relief valve is sticking or kept open, the engine will run too cool and may not have complete combustion. Often a blast or two of wide open throttle before docking, if possible, can burn off any accumulated residue. Has the motor had service recently where it is possible that someone may have put it into break-in mode by mistake?
If this continues, then it would be best to have the motor checked out.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.