These corals are still a mystery to me and many other hobbyist.
For what ever reasons these corals, the frogspawn and hammer corals,
can and will do splendid under almost all environments in captivity.
I have had both types with mixed results.
A very niced branched frogspawn that I had to move after splitting for
over 2 years, suddenly just started to melt away, branch by branch.
Nothing in my tank changed, in fact all of my other corals including some
much more difficult to keep corals, were thriving, but this one just faded
away.
The same thing happened to a hammer coral I had, after a year and a half,
it too started to fade and melt away, but for whatever reason, things turned
around and it has stayed with me now for another year.
Most of what I have read and learned about these corals seems to lead me
to believe that these corals just do not live long in captivity.
I have also found this to be true to the same extent with xenia corals.
They will just fade away for reasons unknown.
Of course, this is only my take, I am sure you'll get many more ideas and
explanations here.
Good luck
