Report it. Report it to the senior. If they do nothing, then report it to the nurse. If they do nothing report it to a manager. Keep reporting it until you either have a feasible explanation why the resident keeps bruising, or until something is done about it.
Communication. In most cases, a person will explain what they feel comfortable with, if something is wrong, where pain is coming from etc. It's a lack of communication that generally causes dissent and interpersonal issues.