What a shame! I’m very sorry to hear about Chloe’s sudden demise. I don’t have a clear idea about what might have befallen her, but it doesn’t sound like it was related to gas bubble disease (GBD). The various forms of GBD normally don’t kill so quickly and cause characteristic symptoms (positive buoyancy, subcutaneous emphysema, distention and swelling) to alert the hobbyist to the problem and allow for treatment to be attempted. Chloe didn’t show any of the premonitory symptoms of GBD and went from apparently fine to lethargic and struggling for no apparent reason to dead all within 24 hours. That’s not consistent with the usual progression of gas bubble syndrome, so I suspect something else was going on.
Acute and peracute bacterial infections can kill seahorses with little warning, and seahorse keepers sometimes refer to such systemic infections as "Sudden Death Syndrome" because the seahorse can seem fine one day and be dead the next, allowing no opportunity to detect the problem and treat for it. Such an infection seems to match what I know about Chloe’s problem better than GBD, so septicemia is one possibility.
I have no idea if if this problem could be related to the red slime algae and the medication you treated the tank with, which produced all of the microbubbles and reacted with your skimmer to produce copious amounts of wet foam. Can I ask you what product you used to try to eliminate the red slime algae or cyanobacteria, Carrie?
At this point, it may be that we should be concentrating on eliminating the red slime algae and whatever is fueling its growth, rather than simply treating symptoms as they crop up in the seahorses. I would be happy to go over some of the measures that are normally useful in eradicating cyanobacteria and other nuisance algae with you if you feel it could help.
Best of luck ridding your tank of the red slime algae which has caused so many headaches and heartaches for you recently, Carrie, and all my condolences on the loss of Chloe.