It sure sounds like it's the yeast that has overflowed.
But rather than your second question, the better question is, how much yeast did it spill compared to what is left in the flask?
Usually, even with top cropping yeasts that overflow the starter flask (or fermenter), the vast majority of yeast is still inside.
What does yours look like, is there a thick layer of similar "milky, creamy stuff" in the bottom of the flask? If so go ahead and use it as normal, you probably only lost a small percentage.
However, in the unlikely event that most of the yeast escaped, repitch what remains into another starter (with more head room this time) so the cell count can grow back up.