oily sludge in expansion tank/coolant

just noticed some oily sludge/ grey in colour in the coolant expansion tank. got me a little worried, water pump or head gasket I am thinking? bikes got 24k ish. nothing in the oil itself, plugs are clean and fine, bikes running great, coolant doesn't seam to be going down. did a search and only came up with this. whats the most likely do you guys think?

viewtopic.php?f=5&t=16820&p=194867&hilit=cast#p194867

I had something similar with my 690 Supermoto at about 10k miles. Got all traumatized, pulled the water pump to examine it, everything looked ok. So, I drained the coolant, back flushed the cooling system completely and reassembled and refilled with distilled water and Redline Water Wetter. 24k miles now and nothing else ever appeared and everything still runs great!