That is probably cambium. It will look almost like light brown paper sack on hackberry, it’s the layer right under the hard orange crumbly layer. It can be paper thin or quite thick. If the back was covered in it, sand it or scrape it off. A little here and there in recesses won’t hurt, but the cambium will fail and can take a chunk of your back with it. If the splinter didn’t go down into the back ring, you are safe. The back ring will be a pale yellow to very white, definitely contrasts to the cambium.