It's not a crack, don't worry. I have two pipes like this and they are called watermarks, where water flowed through the block. A lot of pipes will have them and when the carvers catch it they will rusticate that portion or the whole pipe
I've got an antique WDC meerschaum that has a much larger hairline crack at the shank/bowl junction. It hasn't changed in all the years I've been using it. Unless it starts to leak, or the crack starts to widen, I wouldn't do anything to it. Don't fix it until it's broke.