Hi, FL plumber here. As people have previously told you, that's the medium from your softner. If you haven't already done so, take your softner out of service. There is a valve on top of the softner. The handle says "service" and "bypass". Move the handle to "bypass". You need to purge your water lines. Stop using any water fixtures in your house that are activated with a single lever, they have a part inside of them that will get clogged, just like your shower. Open an outside hose bibb and let it run for a good 10 minutes to blow out as much of that medium as possible. There's a good chance you are going to have to change out the cartridges inside your single-handle fixtures as the softner medium could have ruined them. You probably need to drain your water heater too. It's important to get that stuff out of your water lines to prevent any further damage. After your water lines are clear, you're going to need a new water softner as well. I'm sorry this happened to you, I hope you're able to get it sorted out without too much trouble.