This is spongy moth, formerly referred to as gypsy moth. Apple is a less common host tree for this leaf-eating caterpillar. Once the caterpillars reach that large size, there aren't any effective options beyond hand picking (with gloves). I wouldn't expect spongy moth to do more than nibble on a few leaves on apples. However, if you have 100s on the plant, there is a greater risk of leaves being defoliated.