What is that ?

Most likely just some variegation -- odd pigmentation of no concern. "Suckling" from pests looks different and often has a glistening look around the edges often referred to as 'honeydew.' leaf miners look different, too. you are fine. This is likely older growth or an early leaf. The first ouple sets of leaves can be fugly and never see it again. Old leaves mottle and deteriorate over time, but that doesn't mean they aren't value added... don't remove leaves without an excellent and pertinent reason to do so.