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Well the first responder is right, your coolant should be the orange dexcool, but it may have been changed by previous owner. Not a great idea but others have done so with success when done right.
The screw at the top of the pipe is an air bleed device used to let air escape above the thermostat level. There another at the engine end of your top radiator hose on the right which this aluminum tube leads to the thermostat housing. These are used most often when refilling system and/or when air is suspected to be trapped in system creating an overheating situation. The flange at the bottom of this pipe at the water pump w/one bolt hold down may have a gasket or O-ring I am not sure, I haven't had a problem with mine (yet). The old 3.1 had a 2 bolt flange and a gasket which did fail once or twice over 140K.
Not sure by your statement where the water is puddling, on the ground? if so I would suspect it coming from the water pump weep hole first, indicating a failed bearing seal...this hole is on the back lower center of pump, in this case replace the water pump. If this is on top of pipe area the air bleed may not be tight..be careful these are brass but may be painted black, (soft metal) and is in two parts either could be the point of leak. Also the welds at the T and flange have sometimes failed. Also the clamps on the hose or hose itself may be the point of failure. Best way is to get the system pressure tested at a radiator shop, some guys charge for this, some don't, generally not more than 10 bucks. Hope this helps you some...dave