That's very strange. Maybe one of the SQL Prompt local configuration files has got something wrong in it, so renaming the %LocalAppData%\\Red Gate\\SQL Prompt 5 out of the way could be worth a try. You won't need to reinstall SQL Prompt.
In that directory, RedGate_SQLPrompt_CommonUI_Options_UIOptions.xml has an entry <CandidateListColumnDivider>, which controls how far across the grey text should start. Although the user interface shouldn't let you move the grey text completely out of the way, if something has caused this value to be outside the range of 0.125 to 0.875 then the grey text could disappear. For instance, I manually edited the value to 1.0 and couldn't see the grey text at all.
Renaming the directory as above will reset SQL Prompt's settings, but you could also take a look inside that XML file to see if there is an odd value there.
Project Manager, Red Gate