I agree with Gus68, looks like oil. Was the engine broken in by the builder ? If not it is possible a bit of oil got in the engine water jacket. Flush it, then flush it again. Use hot water, it will help lift the oil into the mix. Clean the cap ( or install new clean one) fill her up, then go for a drive. When cool open her up & see how it looks. Also make sure the oil is new looking any tanning of the oil will tell you there is a problem. Any water will settle down to the lowest point & not show up on the dip stick. If in doubt drain the oil into a clean pan and inspect for water bubbles or tan oil. Good luck Be safe