i don't know seahorses (yet) - but if that was a regular fish, i would say its a bacterial infection and needs treating immediately.a little google research and i found this - which looks about right: https://reefs.com/magazine/seahorses-and-vibrio/
either way - move to QT, lower the temp slowly to 68 (and possibly salinity to 1.020SG), treat with Furan2 or Neomyacin.
