Here's another picture

Could it be that water-retentive moisture gel?

Josh

Polyacrylamide gel.

Did you mix your own soil for your container? Moisture retaining gel could not be in your soil unless someone put it there. It is not normally used in vegetable growing. Al

Looks like the trail of a slug or a snail. Have you seen any leaves or stems that look damaged? If that's what the problem is, you can eliminate them with an organic pesticide like Sluggo.

Thanks everyone - that must be what it is. I looked at the bag and it says "water-holding polymer "crystals" (potassium polyacrylamide copolymer)"

So my next question - is it safe to use this in container gardening with vegetables we're going to eat? I saw the comment saying it's not typically used in vegetable growing. The soil I bought says 'flower & vegetable garden soil with fertilizer' on the bag.

It's safe, but it causes the soil to be more damp. Many veggies don't like to be too wet so you just need to pay attention that it doesn't get too soggy.

I would disagree vegetables like moisture, it's fine to use. Nothing likes to be too wet well except carnivorous plants, nothing likes to be too dry except maybe cacti.
Every plant every vegetable is different, has different needs, requires different soil, and different nutrients.
No one size fits all.

Scroll down to the bottom for info on the gels.

Here is a link that might be useful: Horticultural Myths

Thanks for all of your help - I'm so new to this! Learning as I go...