White Furcula (moth) caterpillar

I have seen a few of these small, split tailed caterpillars over the last couple of months, but this was the greenest of them. When they are resting they tend to hold the two sides of their tails close together. When disturbed, they hold them apart and whip around a little.

This caterpillar was about half an inch long (excluding the tail). The last one I saw - http://www.projectnoah.org/spottings/7246274 - was similar in size. Both were eating aspen leaves.