Water heater sediment from an older unit. This is actually fairly common when water heaters are not drained annually.   If you'd like yours flushed, we have videos, or we can come out and do it for you!   Champion Plumbing 
651.365.1340  
https://www.championplumbing.net 
https://www.facebook.com/3670champion...   BULLET POINTS:

• How Much Sediment Get Stuck If You Don't Drain Water Heaters.
• Why You Need To Drain Your Water Heaters Annually.
• How To Make Your Water Heater Work Efficiently.