If it were me, I might hose it off as best could, then go at it gently with a soft toothbrush and Dakin's solution (http://www.virginia.edu/uvaprint/HSC/pdf/09024.pdf) for a bit followed by a rinse with the hose again. Repeat if it gets noticeably bigger leaving the Dakin's solution on the area for a longer period before rinsing. But this would be an educated guess and nothing more.