I agree with your assessment of the little tube worms — it’s pretty unlikely that they have anything to do with the scratching of your young seahorses.
If you see no sign of hydroids, then ectoparasites are probably at fault, most likely some sort of protozoan parasites. The different treatment options we discussed for eliminating such parasites should work well.
No new news on the book to report. Christopher Reggio, the new publisher at TFH, tells me they are still reassessing the market for a major new book about seahorses. One way or another, I expect it to come out this year.
Best of luck with your itchy juveniles, Seagazer! Keep me posted regarding what treatment you feel is warranted, and I can provide you with additional instructions as needed.