EDIT: If as Bob says above it is the black steel pulley that the belt runs over with the non-grooved side, then ignore all I said below .....

STOP NOW !!!
DO NOT PASS GO !!!
That is the crankshaft pulley ( or harmonic balancer )
If it comes off, all sorts of bad things can happen.
You will need to find a good MB repair shop that can work on a SLK320 and get it hauled to them,
or fix it yourself. ( This would require a new bolt, possibly a new woodruff key and a HUGE torque wrench,
not to mention the special tool for locking the flywheel )