As stated before, underscore (_) is the default active placeholder in a mask. But this can be customized using the slotChar attribute, as shown here:

<p-inputMask mask="99/99/9999" [(ngModel)]="slot" placeholder="99/99/9999"
  slotChar="mm/dd/yyyy" name="slotchar"></p-inputMask>    

The slotChar option can be either a single character or an expression.