I've seen stuff like this in a few different circumstances, but it's usually what the guys mentioned above. Somone has used Bars-Leaks or similar metallic based cooling system sealant on it or the water pump is falling apart.
I'd lean a lot more towards the former.
A cooling system flush is in order for your car. Get that done, refill it with some quality coolant & water and see if the problem comes back. It likely won't and your car will thank you for it. Solids in the cooling system are never a good thing.