This plus sign marker at the bottom right corner of a field means that the contents is bigger than the field can display. It appears automatically on screen, but it will never print.
As an user of the form, probably even using Reader, there is nothing you can do about it.
If you are the owner of the form and have Acrobat, you can make the field a bit bigger or the font size smaller, and then that marker would not appear again (it will, but only when more text is entered).
Hope this can help.
Max Wyss.