If it is anything like the flowerpot, any minor injury will end up leading to its death. It is very obviously bleaching and doing so quickly...if nothing was changed I am looking at that large gash on top of it that might have been the fatal blow. This might not have even occured while you owned it, but it most def is not natural.
I agree with leaving it in and having a nice new piece of LR
. I have several dead corals that now help me out with that. Sorry can't be of any help, but I know the feeling. I have 500 bucks of 3 show quality corals that were nuked in my tank sitting in the sun on my porch that will at some point become bases for future coral. Much easier way to get LR
The only other option on its death would be the cause of your algae issues. I'm sure your nitrates and phosphates are testing 0 right now, but it would be the cause of the growth. Do you run any GFO
or a refugium? If not, it will help that get in check as will weekly water changes after physical removal of some of the algae/scrubbing it.