2 months old is 2 months old, the rock work may have come from a mature sys but the substrate didn't probably and would have to mature from day 1 and the various strains of slower growing bacteria that help mature & stabilize a tank take a long time more then 2 months.
That being said your lighting looks pretty intense for mushrooms also and i have no idea how intense the lighting is at that LFS but it too may be high par over a frag tank as most frags like high lighting and they mix up species in them so to show & sell quickly.
You can test the too new DT theory very easily just pick up a cheap acro frag verses a soft coral & see how it fares. Im betting it will not be happy in a 2 month old tank, i know this because i experienced that in 2 tanks i had with the same issue until my tanks matured, the majority of soft corals i had at that time did ok tho except for, you guessed it the mushrooms, and my Ricordeas were unhappy until i started using Reef salt verses reg sea salt, another thing i had thought was a non issue but they do like Reef params better, not just salinity like say a fish only DT.
Just my exp with this issue it may or may not be the exact same issue but again its easy to test with 1 sensitive frag, its what id do ATM if everything else seems spot on and its not getting better.
Like was stated in a sys with soft corals a lot of warfare is going on id run carbon for sure on it too as well even on a small frag start setup.