You know, it doesn't much matter who/what it is if it's eating your plant. Remove it.

If you want to know for certain who/what it is, rear it to the adult stage. To do so, collect it, feed it the same sort of plant it was on until it pupates (changes to a non-feeding stage) then wait until the adult emerges.

Of course, now that I've explained that, someone will ID the thing. The remedy is still the same: If you want to salvage your plant, remove it.

I'd need to see an improved picture.

That's not a bad picture. A reasonable size, and not as small as many posted here. But yeah, a good sharp close-up is often needed.

The just "Get rid of it" advice above is only 1/2 the battle. Being informed about what it is could help prevent against them in the future, and help be aware of the moth(?) in case you see it. It's also interesting to make a positive ID and learn something in the process.

Anyway, this site is somewhat helpful in identifying caterpillars:
http://www.discoverlife.org/mp/20q?act=x_get_guide&guide=Caterpillars

One possibility is: Trichordestra legitima, STRIPED GARDEN CATERPILLAR, especially if you are on the east coast:
http://www.discoverlife.org/mp/20q?search=Trichordestra+legitima&guide=1

Bugguide.net is another great sight. I bet they'd be able to help

It was said: "Being informed about what it is could help prevent against them in the future,"

Easy to avoid plant-chomping caterpillars of any kind on plants grown for their leaves: Securely cover with row cover.

Not necessarily. It's difficult to put row covers on large plants in containers. For example, I have to put tomatoes in containers (and high off the ground), or squirrels will get to them. But it's hard to wrap the large and irregularly shaped outline of the plants (in the container) with a row cover. And various caterpillars love tomatoes (cabbage and soybean loopers, hornworms, etc.) So row covers aren't always "easy" to use, and all that's necessary. But I agree that they help in most cases, and do offer a big degree of protection.

As Jim said, hard to tell without a better pic - though this one's not bad. Any possibility of a macro against a white background? Close-up of the face and one of the body? I agree it's always best to know what's eating your crop. Hard to fight some pests if you don't know just what they are. And some caterpillars are awesome enough I would raise them just so I could release the butterfly.

My money is on Spodoptera Ornithogalli:
http://bugguide.net/node/view/142846

Moth, nothing neat this time. Definitely get rid of it and start looking hard for more - there probably are. Maybe buy some insidious flower bugs or similar to destroy any siblings.

If I'm right about the id, I find the kin to this caterpillar in my area make good food for resident orb weavers. Just toss 'em in a web. Spiders are my specialty though, not Lepidoptera, so I could be wrong.

Also, these moths pupate in soil and that one seems close to the soil. Seriously consider replanting and getting rid of as much of that soil in the pots now as you can. Why allow more to breed and lay more eggs on your plants. Prevention starts there - and possibly with a sponge of Sc parasitic nematodes for your potted plants. =)

Trichordestra Legitima, named by Jim, is a cousin of Spodoptera. They are both also Cutworm Moths or Dart Moths, just different tribes. Their breeding and pupating habits are the same, so you would want to do the same things regardless of which ended up being correct.

It's also possible that a true ID could not be made from the best possible picture of the caterpillar, and might require an adult from multiple views.