Good question, that is what my tires look like after a track day. If I switch and put them on my trailer and don't street drive them, they look just like that when I start my next track day...
I had a set of R888 and one side looked like they were slicks, then I got lazy and drove them home from the track and it cleaned them out and they looked way better than before, with lots of apparent tread
I don't scrape it off... Curious to see what other folks do as well.