How is current flowing through this P-Channel Mosfet?
The MOSFET has a body diode (not shown on your symbol) which will conduct if source is a lower voltage than drain, that is, the ‘reverse’ direction.
The body diode anode faces toward the drain, and the cathode faces toward the source. As connected, that diode is forward biased.
So, the cap is charging through the diode, even though your starting condition is Vgs = 0.
More about that pesky body diode here: How should I understand the intrinsic body diode inside a MOSFET?