Tridachia crispata is a synonym of Elysia crispata. They're the same thing.
Elysia's point is that this is a sea slug, but it's not a nudibranch, despite the common name. People often call all sea slugs nudibranchs, which is incorrect. Nudibranchs are only the members of one order of sea slugs. All nudibranchs are sea slugs, but not all sea slugs are nudibranchs.
Even if you didn't know what this slug is, one giveaway that it's not a nudibranch is that it eats algae. All nudibranch are carnivores. If you have a slug that eats algae, it's not a nudibranch.