The database available on gems.csail.mit.edu at the time of writing is improperly formatted and will cause MICA to fail.
The headers in the coarse.fasta file are formatted as '> seqname' which diamond interprets as a blank name.
The current version of mica addresses this issue but the database has not yet been updated.
This can be corrected by running the following command from within the database directory:
mv coarse.fasta coarse.fasta.bak; cat coarse.fasta.bak | sed 's/> />/g' > coarse.fasta