Is a yellow caterpillar a moth caterpillar?

Well it depends what kind of yellow caterpillar you are talking about, if the one you are suggesting about is the size and thickness of your thumb, then yes it is, it's proper name is a "Polyphemus Moth Caterpillar". Polyphemus eat maple, birch, willow and several other trees, but are seldom abundant enough to cause any damage. As big as they are, they are really hard to see among the foliage when they're resting. As with many caterpillars that leave the food plant to spin a cocoon, Polyphemus are sometimes seen wandering around in late summer. This caterpillar spins a tough, brown, oval cocoon that you may find attached to the eaves around your house during the winter.