me too... i used to wipe it off right after i'd pull it out at the ramp. but got tired of filthy dirty towels + just smearing the filth around.... now i just wash it off at home with the hose. i usually have to take a brush to it or sponge. soapy water works best... best if i do it sooner than later..... longer it sits with slime on it. the harder it is to get off... usually next day for me.....I always get one every time I take out my boat.
I carry a spray bottle of water and white vinegar (about 3/4 water).
It is a great scum remover especially on a black lower unit.