help with coolant leak

I recently bought a 91 Mustang. Driving one day for about 30 miles, when I parked my car I noticed I was leaking coolant from what appears to be a plug of some sort on the firewall, on the opposite side of where the heating core is. Can anyone tell me if that is actually a drain plug for the core or do I have a major problem?

Thanks!

i have an 87 gt same problem but mine is on the passenger side below the heater core . Same deal looks like a cut off hose coming out of the fire wall leaking coolant . i think its a drain when the heater is gone . So i guess i will be changing my heater core or ripping out my console ,dash about 8hrs labor.