Surface of a tongue is unique in a way, it has tiny finger like projections called papillae. Sometimes papillae over grow followed by food debris, bacteria and fungi getting lodged between them. Most common after an active infection(fever), prolonged anti-biotic usage. Simple form of home remedy would be to use a soft tooth brush on the overgrown papillae and do it gently. If there is persistence and no remedy, kindly get it examined, so that an appropriate topical medication can be prescribed.