How is an adult echinoderm different from a echinoderm larva?

There is no such thing as an "adult larva" , a larva is always immature ( a "baby"). If you mean "how is a sponge larva different from and adult sponge?" The answer is:

1. The larva is almost microscopic. You can't really see it with your bare eye.

2. the larva swims around. an Adult sponge stays attached to a surface for its lifetime and can't swim