You mean you actually see in the cell that apostrophe?
The normal behaviour of excel, when user starts typing the sign "-" minus, is to treat what folows as a formula, and automatically places the "=" equal sign before user entries. (after exiting the cell) To avoid this default behaviour, and to "tell" excel that you want to type a text string in that cell, just start typing with an apostrophe folowed by your text.
The nice part for this method of typing text, is that you cannot really see in the cell the apostrophe, ONLY in the formula bar! It can be seen in the cell only in cell edit mode.
If you can see that in the cell and that cell is not in edit mode, well, that is a problem.
You can try to remove it with SUBSTITUTE function, which completes your formula:
=SUBSTITUTE(IFERROR(LEFT(SUBSTITUTE(A1;LEFT(A1;FIN D("/";A1));"");FIND("/";SUBSTITUTE(A1;LEFT(A1;FIND("/";A1));""))-1);"");"'";"")