That is a pretty tricky question. Can seahorses get ich. The last I read up on the topic, the answer is yes and no. You usually don't see cysts form on seahorses like you would normally see on other fish. This has something to do with their immune system or built in resistance to the parasite...though I don't remember the exacts after all these years. Even with that said, the ich parasite has been found in their gills and other places when tested...though this is all quite aged information as I don't keep sea horses.
So, with that said, no those aren't cysts from ich on your seahorses. I'd still be hyper vigilant due to there being issues in this tank if I'm remembering correctly.
With that said, the solution is always dilution. Water changes with ro/di water will keep parameters in line and pristine as to not stress out the delicate sea horses so that they will keep eating.