Hmmm - 18 hours seems like a long time for it still to be DH leftovers. When you BD and it's not around O time, do his spermies last for 18 hours?

I'm still a novice at the whole charting thing, but I would say if it's clear and stretchy, then it's probably CM. Spermies would more likely be creamy and not so stretchy (in my experience anyway).

HTH