where's the bug?
aliasname {
command
}
when /aliasname is used, the command is run, in your case, it runs /HEY_MUM! , which is an unknown command, unless of course if you have another alias HEY_MUM! somewhere.
if you were trying to greet someone in the channel, it should be:
rem {
say HEY_MUM!
echo -a Hello World!
}