MySQL warm up time after restart because of lost connection
After a bit or research, it looks like the '#sql-xxxx.frm' files are just temp files created during ALTER statements, and possibly other MySQL behavior such as OPTIMIZE. If the mysqld service crashes, those files tend to get stranded. There's a few other stack exchange threads asking how to get rid of them, and to be honest, none of them worked for me. There's one rather clunky one suggested by the error logs, but instead I just deleted the temp files and dropped the original table I was working on. Then I reimported the table from an earlier dump. 

During the course of an ALTER, '#sql-xxxx.frm' files are created. If the alter runs smoothly and completes, the '#sql-xxxx.frm' file will be deleted on its own.