Likely a caterpillar and if small easily taken care of with Bt.

tj

Here is a link that might be useful: Caterpillars on kale

Collect & stomp.
Flick into soapy water.
Feed to the local spider.

What is BT?

Bacillus thuringiensis.

From the web page I linked above:

"There are several options when considering a low impact insecticide. Neem is a plant based insecticide and acts primarily as an anti-feedant. Although insects are not killed quickly, it causes them to stop feeding and they eventually die. Spinosad is derived from a naturally occurring soil-dwelling microorganism, provides excellent control, and does not harm natural enemies. Bacillius thuringiensis (Bt) is a bacterium that occurs naturally in the soil. Bt is specific to caterpillars and must be ingested in order to kill the insect which is different compared to residual products. Because of this, good spray coverage is essential."

tj

Kale is a member of the cabbage family (brassica) so it can be attacked by the same insect. A little green worm suggest either the Cabbage Looper or the Imported Cabbageworm. Both of these can be controlled with applications of Bacillus thuringiensis - Kurstaki (there are a number of Bts available today) which helps control the insects that chew on plant leaves and is quite specific to them if applied early enough in their lifetime since most can get too large for the bacterium to work.