Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh

 

Madhi is indeed najis filthy and must be cleaned from clothes or other places especially places of prayer. But keep in mind that it must be verified first it is Madhi and it should be visible in a way no doubt of something else.

 

So in your case, it looks most likely that you are not sure and you did not verify it was Madhi visually seen.

 

I say that you need not to fall into the doubtful thoughts without any verification. So only if you see it clear and verify it is Madhi then clean it.

 

Almighty Allah knows best.