"Mr Plod" means mr plod is a british and australian informal term — sometimes affectionate, sometimes dismissive — for a police officer, drawn from the character pc plod in enid blyton's noddy books. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.