Looks like cyano, so my guess is phosphates may be a problem if your nitrates are fine.
I've never been able to keep xenia and colt doesn't do so well in my system (finally sold my big colt), but I've had good success with LPS and (especially) SPS. O wouldn't give up on other corals just because of your luck with softies. Try a birdsnest and go from there if you want to try out SPS.. in the meantime, I would suggest finding out about your phosphates.
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