How not to show black mark in upper left corner on cells, data entered by formula

I have 2 columns. Column BC has date i.e. 12-6-1974 (Birthdate). This is OK. This columns data is manually entered.

Column BD has a formula that extracts out the name of the month and displays it in column BD. Formula is: =IF(BC12="","",TEXT(BC12,"mmmm")) This also works properly.

The only problem is that in the upper left corner of all cells in column BD, there is a black mark. I do not want this to show.

Any suggestions