It's a bookmark -- you have bookmarked a file with ordinary bookmark.
https://www.jetbrains.com/help/phpstorm/2017.1/navigating-with-bookmarks.html
You can remove it in the same way how you set it up. Few ways actually:
Select that file in Project View panel and while focus is still there activate Navigate | Bookmarks | Toggle Bookmark
(F11 using Default keymap on Linux/Windows)
Bring Bookmarks screen in and remove any unwanted bookmark from there: Navigate | Bookmarks | Show Bookmarks
(Shift + F11 using Default keymap on Linux/Windows)