Can you explain the blue trail in this picture?

My son took this shot at work, where he is a grounds keeper. At first he thought it was blue string. But when he got closer he could see that it wasn’t. He said a slug had crawled over a spider’s web, and he thinks the “slime” is reflecting the sky?

Seems to me a slug would break a web, but then again, webs are amazingly strong.

Shall we dissect this picture? It’s sure interesting!

(PS…if you can’t open the first link, try this link.)