I would leave it alone for now, and not even wash it off. This is my opinion, but sometimes if the leak is so small, where there is such a small amount of pressure behind it, there are times that crusty stuff can help to keep it from leaking. Washing it away, might get it to leaking even more.

I don't know how much you tow. Usually under heavy loads or stress the engine gets hotter then normal operator temps. That might be when the engine gets hot enough for the heat to have its effect of the manifold. I would check it daily, especially right after to turn off the engine, and see if you can see anything fresh. These are my thoughts, and I hope you can get it solved.