It’s possible some calcium has precipitated out. You mentioned pH 8.4 which is pretty high. There should be mixing directions on the salt mix. I have not used the salt you are using but if it mixes at a dkh of 12 like someone else mentioned then it wouldn’t surprise me to see some calcium carbonate at the bottom of the bucketis it possible the white specs I’m seeing are undissolved salt? I mixed for 2 1/2 hours. just like I had done before when I started the tank. only this time in a 5 gal bucket. so it was a 4 gal water change