bolt hit the motor casing

Yes, I would get that bolt out of there before something worse happens. Due to gravity, the bolt will likely be somewhere at the bottom of the bell housing waiting for a road bump to make it hit the rotating flywheel again. Someone might be able to retrieve it through the starter mount hole with a magnetic reaching tool.

we had a local mechanic take it apart which found that they dropped the bolt and left it cause all others bolts were installled, meaning he had to go back to the bin and retreive another bolt and left that one in there. some people! But now back at the original shop for warranty. I just wanted to know if the bolt that hit the block will do any long term damage?

I am glad you took the effort to have the bolt removed before it could do anymore damage. Your new mechanic would be the best judge if the existing damage will get worse but I don't think it will. Save all service invoices, and get some pictures and documentation from your mechanic in case you need to sue the shop that potentially caused you a "material/monetary loss due to negligence, and concealment of error". Bell housings are made of cast iron and are pretty tough even if gouged or cracked. And cast iron is hard to weld on the vehicle making "welding preheat" not easy. At this point I would just go after the other shop to cover your additional labor cost fixing their concealed mistake. The job they did should be free.