Here's a second photogram, now focusing on the sub's upper parts.
Here's a link to a short video about the beacons in the mast.
Beacons and backups are important for the pilot's peace of mind for at least two reasons: a delayed ascent, combined with a fast-developing tropical storm, could make D-Chall hard for Mermaid to find. That would be bad, because the pilot can't get out until Mermaid's crane has lifted it from the water.
Note that the green shell is not syntactic foam, despite many online diagrams to the contrary! I feel safe in guessing it's a fiber-reinforced plastic, or FRP. The hulls of racing yachts use carbon-reinforced fiberglass, sometimes laid up as a sandwich of two outer FRP layers with a foam filling.
From what I can tell, the syntactic foam known as IsoFloat is visible on D-Chall's exterior at only a few locations, where it appears as blocks of white plastic. This looks like one of those locations:
Another angle showing the high-tech foam around the hatch can be seen in this screen grab from a video on the website: