It’s possible to get small patches of mould forming on the surface of your sauce. As long as it’s just on the surface, it’s not a problem. It’s actually not your sauce that is mouldy in that case, it’s dead bacteria that have floated to the top. Just carefully scoop those patches out of the jar. But make sure you use clean utensils. This way you’ll avoid contaminating the inside of the jar with unwanted bacteria.