How to splice grounds in a metal box with a 30amp and 20amp circuit
Splice the ground wires together and to a box ground pigtail

You'll need to splice the ground wires coming into the box to each other and to 10AWG pigtails to both the receptacle ground terminal and the box grounding screw, as per NEC 250.148.  That way, there's a direct path for ground current from the MC's ground conductor to the ground wire in the FMC, instead of sending ground current from the MC cable through the box and the yoke of the NEMA 14-30R in order to get to the ground wire in the FMC (or equivalently, through the box and then the 5' of FMC to get to the next box).