[giftbestcom]

Can you explain what you mean by a 'dirty' shinai?

And you could bring you men up to them if they are sensei and ask, "Somehow this stuff got on my men and I am not sure what to use or how to clean it without doing anything bad to my men... do you have any suggestions?"

Phrasing like this makes it completely innocent and not confrontational. Then you just have to hope that they realize there is a problem and they can fix it without you stepping out of line.