You don't say where you're located, or where the bike has spent time before you got it, but chances are good it was in a humid or wet area. What you're seeing is a mix of water and oil. It can get there from condensation, or from a bad speedo boot. Take a peek and the boot and likely you'll find splits or cracks that'll let water in.

You can get rid of most of the water by changing the gearbox oil. Ride it with the new oil until it warms up, then change it again. You might be OK, but if it sat for a while with that contaminated oil in it, chances are pretty good you'll have damaged gearbox bearings and/or pitted gears. You might be able to get one of those little endoscope things that work with your phone and take a look in there through the fill port on the driver side of the transmission. If you see a line of rust, especially crusty rust, it's probably time for a rebuild.

It is really hard to say. Water in the the gear box is never good. Has the bike sat outside for a long period of time? What is the condition of the boot that seals where the speedometer cable enters the transmission. How long has the water been in the transmission? The cautious approach would be to disassemble the transmission. Clean and inspect it. Another choice would be fresh oil in it. Ride it long enough to get it hot and drain. Repeat until oil is clear. After that be sensitive to new noises and check it again in about 500 miles

Thank you, gentlemen. We don't have an arid climate in Wyoming, so I will check your suggestion on the boot. I noticed that boot looked gnarly, so I'll replace it.

I had already planned another oil change as you suggested.

Water in the oil. No doubt about it. It can be anywhere from chocolate brown to a ghastly green that looks like smushed caterpillar juice.

Additional Thoughts

• In the old days it came from condensation inside the cases after parking the hot motorcycle in a humid environment. These days it's far more likely to be the result of pressure washing. The high pressure water shoots in places like the gearbox breather and around the speedo cable when people aren't careful. 
• The same thing can occur with engine oil, but the engine is FAR more likely to be taken out and ridden hard enough for the oil to reach a high temp. When this occurs, the water will "cook off" and thus exit the engine through the engine breather system. Obviously, the gearbox, drive shaft AND final drive are much harder to get up to temperature. It's NOT going to happen with a short 50 mile ride.
• So what the green oil is saying is that the previous owner did not have a good service plan. In order to turn green, the water had to accumulate over an extended period. It means the oil was not changed on a regular basis. And if he wasn't changing the gearbox oil, then he most definitely wasn't changing the brake fluid, battery cables, rear shocks, fork oil, etc, etc, etc ! So green oil could be the "tip of the iceberg".
• I'm very glad you discovered this by taking to time to actively change this oil, but you need to understand that this could be the lead up to having to change out the bearings inside the gearbox. Ball bearings are highly susceptible to moisture when the oil/water combination is allowed to sit because (unlike roller bearings) the outer race in effect forms a cup that holds the water in contact with the bearing race long enough for rust to form pitting. The pitting chews up the precision balls. The precision balls then start to make dreadful noises. Not good.
• All of this is why you MUST change your gearbox, drive shaft and final drive oils at least once a year ! It doesn't matter that you only put 35 miles on the bike last year !! We do not change these oils based on mileage. They are changed according the calendar exactly because of water accumulation !!! If you believe that buying 1.3 quarts of oil is an excessive expensive after 35 miles, then wait until you get the bill for a final drive rebuild !

Hope this helps.