This should be “Owner Files” when the Excel file in question is still open or these files have not properly been deleted before:
Owner File (Same Directory as Source File)
When a previously saved file is opened for editing, for printing, or for review, Word creates a temporary file that has a .doc file name extension. This file name extension begins with a tilde (~) that is followed by a dollar sign ($) that is followed by the remainder of the original file name. This temporary file holds the logon name of person who opens the file. This temporary file is called the “owner file.”
So as @victor_palacios has suggested you would best filter them. It is possible that a new version of KNIME would automatically ignore them.