No diary = no information! Your lighting is also a detriment to any GOOD guess ... my first GUESS was that you were doing an outdoor grow and it was some bird poop.. I don't think it has anything to do with nutrients being spilled on your leaves... the pattern just doesn't suggest it at all - nor does the color IF it's as white as the photo suggests. Somebody painting something and splashed paint? Sigh... really, without good photos and without a diary, we're in the dark. Sorry....