what are these brown specks coming off my dog?

I noticed this only from giving him a bath. This time I decided to bathe him myself as I usually take him to the groomers. There was a collection at the drain and I tried the paper towel test to see if it was flees but it was clean (no smears or red). As I blow dry him and brush him more has come so I’m not sure if this is dirt or dead skin or fleas. Sorry if there’s already a post about this, I wasn’t able to find any answers or anything close to this. This also didn’t trigger him like an itch and this is mostly in his butt area. He had also been taking simparica for the past few years.