[MondoBongo]

usually some elbow grease with a scraper and vinegar do the job for me.

if you want to up the game, hardware stores sell Muriatic Acid for etching concrete. might be worth a try, but keep in mind that it is nasty stuff and you need to take precautions before using it (eye protection, dilution, skin protection, nitrile gloves, work outside, etc).

conversely, leaving it there won't hurt anything, and eventually it will get covered with new coraline.