vibrating belt pulley

[SG_C280]

hi everyone. i have a 1999 c280 (m112) that lately has a grinding noise from the serpentine belt area in cold mornings. but this morning the noise was a lot worse and didn't go away. i took a look at the area and one of the pulleys was vibrating noticeably, and when i turned the car off it had some play when feeling with my hand. what is the pulley in the picture called, the one where the arrow is pointing (has 4 bolts around the center outing)? can i drive the car like that for a couple of days? am i looking at a big bill? my mechanic is out of town until wednesday. any suggestions would be very appreciated.