That "dust" looks suspiciously like red dye someone may have added to AC lubricant/refrigerant. You might check under the AC low side cap. There will usually be some residual oil under the cap. If it's red, you know you have a compressor rotating shaft seal that is leaking your refrigerant/lubricant. Some shaft seals are replaceable. Check w/ ackits.com for shaft seal, and you may need an installation tool. I've found it is easier and sometimes cheaper to find a good used compressor. check car-part.com.
BTW, this part was likely broken when someone tried to adjust belt tension w/o releasing the lock nut on the sliding mechanism.
See you down the ROW!
1999 Subaru Forester