Depending on how well everything was washed/degreased, i.e the block etc, there may be some settlement of oil and shite in the water jackets within the block.
When you obviously get it up an running again you will end up with a small amount of oil/degeaser in your cooling system.
If this were the case, there shouldnt be much creamy stuff in there and a good flush out of the cooling system should get shot of it (if it is that of course!)
If it aint bad, give it a good clean out with lots of clean water and try again. If it doesnt work, it could be the h/gskt