Yes, its possible but you have to rely on RSTP itself to block one of the links, as you know there is no link aggregation on the MX devices. Although the MX doesn't participate in RSTP it will forward BPDUs.
Configure another port on the MS355 stack the same as the one currently connected to the MX, and likewise on the MX configure another port that is identical to the one connecting to the MS355 stack, and then put a cable between them.
BPDUs will be sent up the links, forwarded by the MX, and then down the other link back to MS355 stack. On receiving its own BPDUs the MS355 stack will block one of the ports based on the interface/port number (which goes towards determining the superior BPDU), so one of the ports will be blocked, and the other will become designated. Then if you experience a failure BPDUs will cease to be received on the blocked port and it will move into the designated state.
Couple of points: the ports will ideally need to be trunk ports on the MS. Ensure RSTP is enabled on the MS. If they are access ports then they'll run PortFast by default which will cause an initial issue, but it should recover. You shouldn't need to run Loop Guard or Root Guard, and you definitely don't want BPDU Guard on the ports.