It may be time to replace the cartridge for the Moen shower faucet. We're thinking it may be time to replace ours as well as we just replaced the one for the kitchen sink as it was leaking during a "Pre-flight" inspection recently. Over time, a little grit can get into the "O-Ring" on the cartridge and that's all it takes to cause the leak. Pull the cartridge and take it to a Home Depot or Lowes for a replacement or just order it from Moen. Ask if you have a lifetime guarantee and if so, they may send you a new one fee or at the very least you pay the shipping charges. Good luck.
In dog years, I'm dead!