+1 on not enough light and being new to your tank.
I rescued an acro from someones tank that was completely bleached. I stuck it in my tank and within a few months it took on the craziest colouring (I've never been able to ID it). However, about a month and a half ago I moved and set-up a new tank and the mystery acro temporarily got relegated to a dark corner of the tank. Between the move and the low light it went so brown that in the low light it looked black. I have recently moved it back to a high light area and it's colour is slowly coming back.
Corals can regulate their zooxanthellae levels depending on their needs. In low light they need more algae zoox (which are brown) for energy, so they up the zoox level and look brown.
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