It's called seepage. You sight glass is on it's way out (literally). Replacement is really easy, it's just the cost of the sight class that gets you.
Repalcement: Either drill the clear plastic, or break it. If you drill it, use a screw in the hole, and pull out the assembly. If you break the clear plastic, leaver out the whole assembly with a large screwdriver. Clean the surfaces on the engine. Lightly coat the new sight glass with oil, and tap it in with a large socket, or a short section of PVC pipe (either need to be the size of the sight glass).
Oh, this easiest done when doing an oil change.