Please, guys, stop!!!!! I think his question was very specific, and he seems to have given all the information necessary. He wasn't asking how to clean a ferrule. He was asking about a specific spot that wouldn't come out, a spot that came up just after he had a tip put on. Because he says he has tried to use soap and water, we can assume that the rest of the ferrule came pretty clean.
This type of spot comes up all too often, especially with fiber ferrules. It's a pre-cleaning issue and has been addressed very well by Macguy. It's glue.
Fred