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ITs really hard to tell, but it looks like it. Can you get a closer shot? It looks grey from the photo.
Chances are you wont be able to save it. Or its not worth it, if its just on top, you could cut it in half and just grow the rest of it, but it might expose it to contam. If you do decide to cut it, put sterile moist vermiculite on top... to act as a seal from contams.
If you want to see if it is... take another pic of that quickly, if it is a contam and it sporulates, you may get the rest of your terranium contaminated. Maybe try a bleach water solution... not tooo much... but just enough. And spray the sides of your terranium... and lightly spray the cake.