Hi, I'm a wildlife rehabber, I specialise in grinches! It's quite possible your grinch just decided to make himself at home under your house because it's the only available shelter. They typically prefer dark, mostly enclosed remote locations that still allow them foraging access to urban food sources, but as the urban sprawl progresses, this is less and less frequently your classic lair on a mountain and more and more often just a place the grinch calculates is relatively undisturbed. How often do you look under there? Try putting some festive lights on under there for next time.
While we're at it, this is a great time to mention proper storage for household waste. A grinch can't be tempted by food a grinch can't smell ot get to! If you don't take steps to prevent it, a different grinch could move in when this territorial vacancy appears. Or worse, your he could be a she — in which case you should definitely give wildlife services a call, because you can't remove a pregnant, nesting grinch from your crawlspace safely by yourself.