Text box linked to cell value

First remove the formula from the TextBox and make sure that it is just text or blank

If the Chart is in its own Sheet eg "Chart1" then:

 Insert into a macro:

          dim A$
          Sheets("Data").Activate
          A = ActiveSheet.Range("J7").Value

          Sheets("Chart1").Activate

          With ActiveSheet.Shapes("TextBox").TextFrame
                 .Characters.Insert (A)
                 .Characters.Font.Color = RGB(0, 150, 0)
          End With

     Note: You'll need to 

     replace "Chart1" with what ever is on the Chart Sheet Tab

      replace "TextBox" with the name of the specific TextBox you want to format.  You can find this out by hovering the mouse over the text box and the name will pop up in a yellow box.
     
     Modify RGB function with the (R,G,B) where R G and B are integers 0 to 255 to define the color eg, pure Red would be (255,0,0)

If "Chart1" is embedded in a Sheet with cells eg "Sheet1" then:

 Insert into a macro:

    Dim A$
    Sheets("Data").Activate
    A = ActiveSheet.Range("J7").Value

    Sheets("Sheet1").Activate
    ActiveSheet.ChartObjects("Chart 1").Activate
    
     With ActiveChart.Shapes("TextBox").TextFrame
         .Characters.Insert (A)
         .Characters.Font.Color = RGB(0, 150, 0)
     End With

Hope this helps!