Hi,
    Don't use the pen if there is any way of avoiding it. It sounds like
there is
excess protein on the slide. (I am assuming it is from the pen, you didn't
say what
you were using for slides) Usually if there is a proteinaceous film on the
slide,
often from an adhesive (albumin, gelatin etc.) the protein will collect the
chromogen and develop a haze on the slide. If there is a protein content in
the pen
that may be your problem.
good luck :-)
Amos Brooks