I have a 1997 8 hp merc, It leaves the same residue droppings all over my concrete. I checked the lower unit lube, it was full and no water intrusion. I realized it was unburned 2cycle oil mixing with water. If you want to test this to see if that is the problem, try using synthetic 2cycle oil and see if the problem disapears.
I have the same problem with my 1996 6hp merc. It is from unburned mixture. Possible cure is to add the thermostat kit which will raise the temperature in the combustion chamber enough to burn off some of it. Looks ugly , but doesn't seem to hurt performance. I'm adding the thermostat because I don't like what it does to the water.
I had an 86 Merc 6 HP that discharged unburned oil for about 12 years. It will eventually plug the exhaust ports and you can suffer loss of power and hard starting. It just needs to be cleaned once in a while, depending on the hours you put on it.