Yup! Know it well!! First, get an old toothbrush and gently scrub the chin with warm, soapy water. Rinse well and towel dry. Get some acne pads and use those, or take some 10% Benzoil Peroxide (any CVS or drug store has it in the acne cream isle) and but it into the chin. It'll clear it up. Keep doing it till the chin clears up.

Actually, acne is more prevelant on cats with white chins or totally white because colors hide things.