Bro, I had similar experience as you.
Some time back, I was feeding my tank a mixture of poly lab reef rod and phytoplankton about one/twice a week. All I had in tank at that time was, a couple of clowns, hammer, octo coral, zoas, yuma-mushrooms, jewel/gonio and a xenia frag.
The next day after feedings, my mushrooms would shrink and spew black stuff out from it mouth. I thought it may be just expelling excessive waste due to the rich feeding, so I just continued the practise. However, 2-3 weeks of feeding later, I lost my hammer, followed by octo then jewel and lastly the xenia, all died in a span of less than 2 weeks.
I do not know if it was the feeding, or something else. But I just stop feeding the reef rod & phytoplankton mixture since.