Red Sea Blue Bucket has a long and rich history of leaving brown crud in your mixing bin over time. There are an extensive number of mentions of it with pictures on threads here on R2R.
I'm not suggesting anyone should avoid RS salts though, I'm just noting that if your hope is to avoid brown stuff of unknown makeup, you should choose a synthetic salt like HW Wiegandt or Brightwell Neomarine (or Tropic Marin once they're straightened out).
The brown stuff takes a while to build up with the RS salts, so it will take a while to begin seeing it in your mixing bin. I used RSBB for several years but got tired of draining and cleaning every couple months. This is going to likely be the case though with any salt mix that leans heavily on mined materials or evaporated seawater.