I'd not worry too much about the power washer, the manzys are rather flexible actually. You can blast the lower parts and the trunks etc, certainly the larger pieces. I leave those stumps in the ground unless I can easily pull them out by hand.
There are lots and lots of those type of stumps all over the place along road sides, but it takes work to dig them out.
After about 1-2 weeks, the plecos, shrimp etc will pick off the grey color.