web-like growth on dwarf crab apple
This is the webbing of the Eastern Tent Caterpillar. They are very common spring pests in our area. The caterpillars eat leaves within the tent and slowly grow larger.
They don't usually hurt the tree because the leaves can re-grow. However, consider either knocking it down with a broom, or simply opening it by tearing it apart with a stick so that birds can come and feast.
You can also prune it out and put it in a bucket of soapy water if you like.