Sludge in expansion tank

Hey guys, just noticed this morning there was sludge formed in the expansion tank?
There is no loss of coolant or oil and the coolant was changed in August. Any ideas what it could be. My car is a Zafira B 1.6 petrol. Many thanks.

http://i949.photobucket.com/albums/ad339/ambia112/E66F0732-58A7-40DF-A9B0-B810F4410C02.jpg