Mine will look white when it's dark, I see you have them with a blue tang, how strong is the current in the tank? And the temp of seahorse tanks should be around 72. That can make them change color as well.
Horses are best in species only tanks. Low current and cool temps. Most people run chillers on horse tanks. There actual temp range is 65-68. Too cold for most other sw fish . Exceptions are pipe fish and draggonetts. In warmer water there prone to bacterial infections