11 days after tooth extraction, clot fell out and bone was exposed; now I see cavity-like marks on it and it mildly hurts. Should I be worried?

Probably not. After 11 days, your gum tissue has most likely closed over the socket area to a point that the risk of dry socket is minimal. Since you are not swollen and the pain is mild, it seems you are on the mend. For peace of mind, you can always call your surgeon for a follow up visit.
Post op issue. Have it looked at by the dentist who extracted it. This is normally considered part of routine free follow up aftercare.
Post extraction. If after 11 days you are now having pain and the bone is actually exposed, you need to see a dentist right away.