Okay. "Grossness" may have a double-meaning here. "Gross" can be a negative sounding term meaning "indecent, vulgar, or unrefined," but it can also mean "the entirety or whole of something before deductions," as in "Gross annual product," the total amount of money you make in a year before taxes are taken subtracted from it. So, the poet is using two meanings to form one image with one word: a "measureless" entirety of something that is unrefined like "slag" (which is cinder or the refuse of burnt wood). "Nestles" is an active verb here that describes the position of an object. It is being used to say that "the seed perfection" is not out of place or inappropriately beneath this "measureless slag." Rather, the seed's presence there is fitting and comfortable because it plans to sprout up from the "slag." The word "nestle" uses the word "nest" which is often used as a sign of comfort. That is why there is a Swiss company called "Nestle" which is known for products like hot chocolate, and why welcoming, homely cottages are often described as being "nestled" among trees. Hope this helps. It's probably more analysis than you wanted.