Do you have slugs around there? is there any evidence of something chewing on the leaves.

Slugs leave a slime trail. Look around the ground nearby and see if there is some on the ground.

Or could it actually be a spiderweb with dew and the sun shining on it. It's hard to tell unless you are looking at it

It could be "honey dew"--a fancy name for aphid excrement. Is it sticky? Do a google search on aphid honeydew, and see what you find.

Hi ,
You are right. I saw 1 slug this morning. I removed it and threw it out of my garden.
A few days ago, something was eating my yellow cosmos leaves. I did notice that 2 of its leaves had some kind of dried white liquid on them. I moved it to another part of the garden. Nothing happened after that. Could the slug be eating this?
How do I control the slugs?