How do I grep on a live log (stdout)?

I'm not sure if that what stdout is, but that's what I think.

What I want to do is filter out a certain log message when running an executable.

I tried this:

Executable 2>&1 /dev/null | grep -v "fixme:quartz:Parser_OutputPin_QueryInterface No interface for {56a868a5-0ad4-11ce-b03a-0020af0ba770}!"

And a normal:

Executable | grep -v "fixme:quartz:Parser_OutputPin_QueryInterface No interface for {56a868a5-0ad4-11ce-b03a-0020af0ba770}!"

But the Terminal was still filled with thousands of fixme:quartz etc. messages.