That's OK if you mount the disk read only, you can do that with UD, don't mount read/write or some data will be changed just by mounting/unmounting and that can cause issues.
Looks like it dropped offline, you can try power cycling the server, if the disk comes back online run the command again (check the drives identifiers are still the same, if not adjust as necessary), use the same log file and it will resume.
Not sure what the problem is but you can use the console instead, also don't use -n or nothing will be done, with the array in maintenance mode type:
xfs_repair -v /dev/md7
Without parity you can add it to the array to an empty slot without issues, if you want to add it to a previously assigned disk slot you still can but need to do a new config first.