We also use CE for our site CMS and are experiencing the same problem.  We use in-page editing, so the users are able to change a field on a web page in place.   It seems that in these cases the span tags are being added when the text is a different color that the default color of the body of the page.  Each time the field is editted, CE adds a new set of spans,, but (oddly) only if you leave the page and return to it before editing again. 

 

See the image below. The <span style="color:white"> tag pairs are NOT in our CMS, and so should not be part of the editable text in CE.

 

Note: I will also upgrade to 6.4 to see if that helps.