Yep, that's what it looks like. Must have been a more humid day outside than usual.
Some incoming air crosses past the cold air conditioning evaporator coil in the heater box. As the air is lowered past it's dewpoint ...and then the moisture condenses onto the cold coil, just like water forms on a cold beer or can of soda outside. The water builds up and runs to the bottom of the HVAC box and drains out that tube.