Be sure to check your air humidity as well. If your humidity dropped very low, sometimes it can kill off the surface growth. Try putting a cover on it if your air is very dry; you'll need to acclimate it over some time.
Also, utrics in general tend to be a bit sensitive to minerals or other impurities in the soil and water. I noticed that you're using what seems to be a porcelain dish. If it isn't glazed on the inside, then that could also leach crud into the soil and cause die-off.
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