I don't know if this is the same animal that Serena saw, but it does have very large eyes, a feature that she remembered.
At first I thought this might be an isopod because it looked fairly flattened from top-to-bottom. However, with more attention, and some research, I'm now thinking this is probably an amphipod. Although amphipods are often flattened side-to-side, some of the hyperiid amphipods can have a more rounded appearance.
I didn't have a lot of time, but I also found one reference that listed a few known associates of pyrosomes which mentioned hyperiid amphipods.
For now I'll call it an amphipod, but I'd be very interested in feedback from anyone with more experience.
And now for some more imaginative thoughts:
What is it like for an amphipod to live inside of a pyrosome? When I saw one, it was positioned such that it appeared to have a view looking out at the world through the transparent tunic of the pyrosome. It must be quite something to be drifting along inside of a long, jet-propelled cylinder. Do they see other animals swimming by? Can they move to different places within the pyrosome, e.g., from side to side or front to back? What is it like for the amphipod when the pyrosome starts to bioluminesce? Pyrosome bioluminescence can be triggered by motion. Can the movement of an amphipod start a bioluminescent wave?
Above I mentioned that Serena remembered the amphipod's large eyes. While observing them, I tried to come up with a name for their color. I'll let you look, and then I'll tell you what I came up with below.
You're going to laugh at me, but eventually I decided on "marsala"— the 2015 Pantone Color of the Year!
What? You hadn't heard about the color of the year yet? Well then, you'll have to check it out here at the Pantone web site and see if you think it's a match for this amphipod's trendy eye color.