threat

Hello there folks! I was wondering if someone can tell me what this is? It has its own mouth and a seemingly transparent, quite long, body. (BTW, if it helps, it retires behind the rock on the right at night) Since i cannot tell searching through the internet i hope someone can take his/her...

We recently added a white clawed hermit crab, now named Sheldon, to our reef tank to help battle a hair algae problem. It’s a similar size to Dug, our tiger conch. However… not that I see them side by side, I’m concerned. Sheldon’s larger than Dug, but not by much. There’s not a lot out there...