That's a butterfly caterpillar. They curl the leaflets around them for protection from predators and then eat the leaflet. When it's mostly eaten away they move to a new one and start again, until they eventually mature. I don't worry about them, but I have a lot of palms and some chewed leaflets don't matter.

Yep. A palm dart caterpillar. I look out for them because they ravage my solitaire palm. they end up as a snack for my chooks.