A generic answer... You need to clear screen by calling background(colorWanted) at beginning of draw whenever you want the screen cleared. Or draw a rect() filled with the color of background over the text to "erase" only that area.

" I'm not sure if is text() that makes the conversion..."
Actually, Processing has different text() functions (methods) for each kind of parameter that we can give it. So it has a text() function were we give an int (or a char, a float, etc.) and it converts the input to a string before calling the common version using strings as input.

Ok, so how would I do this if I want to do this on a changing background, which in my case is camera feed ie capture()? Would I call image() again in a redraw when my new data is ready(comes once a second)?

Thank you! Just figured that out myself and it's working good now. Now just to do some Serial debugging...