No, don't hose it off. Yeah, this one's common. Direct water can wreck the motor and electrical parts.
The right way is to remove the battery first. Then tip the mower on its side, making sure the air filter and motor housing are on top so no oil leaks into them. Use a brush or a dry cloth to wipe away grass clippings and dirt from the deck, underside, and the blade area. You can use a damp cloth on the plastic surfaces, but avoid any electrical connections, the battery port, and the motor.
IF THAT DIDN'T WORK: If it's really caked on, let the debris dry completely, then brush it off. Never use pressure washers, solvents, or gasoline.
HEADS UP: Always store it in a dry place. Water and electricity are a bad mix, even for a cordless tool.