how the pickups work
These fall from enemies you hit while wearing the full Nebula set, and they hang in the air until you get close enough to collect them. You cannot store one for later or save it for a boss, so the armor quietly pushes you toward fighting at close range instead of kiting. Collecting another while the buff is running adds a level on top, and the levels expire one at a time rather than all together.
the catch
Extra regeneration only helps while natural regeneration is allowed to run at all. Poison, fire and acid all suspend it, which cancels most of the healing at exactly the moment you wanted it. What you keep in those situations is the tick damage being shut off, hunger aside, so this is better understood as a shield against damage over time with some healing attached.
in multiplayer
Anyone can pick these up, Nebula armor or not, and each pickup spreads a level to players standing nearby. On a team it only reaches teammates. One person in the full set can keep a whole group topped up as long as everyone stays close.