1
Shortcut via toolbar
These are spaces and other characters like tab and line break. There is a shortcut in the toolbar that enables/disables this setting. The button is that performs the action is in yellow (the third on the left to direct) on the image.
Via preferences
Another way to hide/view is through menu window>prefereces
. In the three view left go on General>Editors>TextEditor
. Clear the option show Whitespace characters
as in the image below: