Probably condensation. The same junk, like tapioca pudding, was in my breather hose from the passenger side valve cover. Maybe the cap wasn't on tightly.

Either way, I noticed the same thing and so did many others in the hose out of the back of the valve cover.

The only other concern is if it was anti-freeze mixing with the oil, but you wouldn't find it there typically. So check the anti-freeze level, and if the oil on the stick looks clean, not to mention if he is getting any white smoke out the exhaust...