Your location and host plant would help.

The basic answer is: Probably a moth caterpillar, but there are some fly larvae that look similar also.

Caterpillars are sometimes found away from their host plant, but if you actually saw this bug eating that leaf, it would be nice to know what plant the leaf was from.

Looks like a rose slug (sawfly larvae) to me. A hymenopteran.

Here is a link that might be useful: {{!gwi}}