When I got my driftwood I was advised to soak it in hot water over and over again to get it really clean. I don't think it had been in anyone elses tank but I got loads of red colour out of it. If the bit you've got hasn't been in the water for a while where has it been? - could be full of dirt/anything really.
I'd give it a good soak and wait until the water was clear before I put it in my tank - or maybe chuck it out and get a bit I knew to be safe for the fish.