grace note

In musical notation, a grace note is distinguished from a regular note by print size. A grace note is indicated by printing a note that is much smaller than a regular note, sometimes with a slash through the note stem (if two or more grace notes, there might be a slash through the note stem of the first note but not the subsequent grace note).

You can see an example at the very upper right corner of the sheet music in Danbooru post #2863696.