What kind of liquid can be seen 'leaking' from the bottom of a Boeing 737-800 engine?
There is really no telling without being there &amp; getting a sample of the fluid.  A mechanic can tell the differnce between hydraulic fluid vs engine oil vs fuel vs etc, but the place where any of them would come out is essentially the same spot under the engine.

In general, any of the above is uncommon but far from unheard of, and in small amounts is no big deal.  The fittings aren't supposed to be leaking, so a mechanic will probably end up fixing whatever it was, but the hazard from a few drops just isn't all that great.  Sure, the idea of "oh, just a fuel leak, no big deal" intuitively sounds pretty bad, but the fact of the CFM engine is that the motor is incredibly tolorant of this sort of thing with nothing bad actually happening.  It needs to be checked out because something isn't exactly right, but there would have to be several other things go wrong as well (each of which is pretty rare) before anything bad actually happens.

Some lines are allowed no leakage at all, while others actually do have a "drops per minute" limit for what is allowed without needing maintenance action.  Not being a 737 mechanic, I couldn't tell you offhand which fluids are in which category.  If I see it leaking, I call the mechanic &amp; he can sort it out.

Of course, in the days of the big radial engines, the saying was that when it's leaking, that means there's still fluid in it, and when it stops dripping, THAT is what's bad, because that means you don't have any more oil!

We're a long, long ways from those old days!