Seriously, have you contacted the landlord with your concerns? As far as I know, all states have a sexual offender registry, and offenders are required to report. It's online. We all make signs, or print graphics. We are not law enforcement or lawyers. Find out who he is (don't check his mail, your on camera), maybe you have a cop friend who could help with background. He could also advise you of what Not To Do a lot cheaper than a lawyer. From a legal standpoint the neighbor has not broken any laws, that you know of. You can and should follow your instinct, and pursue more info, but tread lightly. You don't want to end up on the wrong end of a civil suit.
But you are Texas, it might be fun to shoot the mower.