The tank is "glass lined" meaning there's a coating on the inside of the tank that's similar to the old "blue granite" coating on stamped steel cookware. In the owner's manual for Suburban HWH's there is a statement that sludge and minerals settling in the bottom of the tank will insulate the bottom of the tank and cause it to overheat. Overheating will damage the glass lining (crack it) and what you've found are probably bits of the glass lining from the tank. I'd watch closely for leaks around the HWH. If you have a borescope, or a small computer camera, you may be able to insert it into the anode fitting and inspect your tank, otherwise, just keep an eye on it. Replacement tanks are available, but unless you're "quite handy" it's probalby easier and likely to be cheaper to just buy a replacement hot water heater.
If you're interested in getting a "borescope" there's one available at Harbor Freight for about $75 or you can get a "pencil computer camera" like the on in this link.
http://www.ebay.com/itm/7mm-Inspecti...item4ac0b0deee
Keep in mind that the EBay borescope is connected by a very flexible cable and is difficult (not impossible) to manipulate once it's inserted in the anode fitting. Inspecting the inside of the tank is not mandatory, but I'd almost bet you've got a cracked glass lining.