That looks like a washer from the orange handled shipping bolts that hold the gantry still. You could have dropped it when unpacking or perhaps Flex dropped an extra one in there during packing.
Thanks for asking @Clone! I agree with @markwal. It looks like the rubber O-Ring that comes on the two red knobs which secure your laser arm in place during shipping.
Would you mind checking each screw and letting us know if one of the black, rubber O-rings is missing?