I'd say it's probably fixer related. If it is, then the prints are ruined, the paper has printed out and you can't remove that stain. I've noticed that unprocessed Fomaspeed (RC) papers left out of the box will get bluish gray after some time, but haven't tried it with FB paper. If you have selenium toner, try mixing few ml of 1+9 solution and put a drop or two on the border. Leave it there for 2' and if it leaves a stain, it's a sign that halides were left after fixation.