"xenia crash" can someone see the problem in xenia picture i posted

[Black Phantom]

Sorry to agree but your tank is way too small for those tangs. What other corals do you have close by the zenia. They may be getting stung or under a chemical attack.
I can't have Zenia in my big tank as they just shrivel and die, but they grow like weeds in my 50. Exact same water just different corals.