I’m not quite sure I understand your question about parasites “on any plant or fish” being white, sir. I know of no parasites that attack marine plants and ectoparasites on seahorses will be killed by freshwater dips of suitable duration or treatment with hyposalinity at an adequate level. In that event, the ectoparasites are killed because water will move into the bodies of the parasites via osmotic pressure until they rupture or lyse, whereupon they typically drop off of their hosts…
If you are observing ectoparasites embedded on your seahorse and they have changed color noticeably, becoming white, that could be an indication that they are dead or dying but you would need to examine them under high magnification in order to be sure of that.
Pete Giwojna, Ocean Rider Tech Support