Jim, we, as home inspectors, want to do the best job for our customers as we possible can. We want to be as honest and up front as we can and not over step our bounds. We are home inspectors - not mold inspectors, not pest inspectors, not structural engineers, etc. If we start making statements that are outside our profession we can be putting ourselves in a position that for the most part we don't want to be in. Tell your customer that you found what appears to be a mold type substance at or near the supply vent in room whatever. You can, if you feel comfortable, make some suggestions as to whom they may contact. For the most part leave it alone with the fact that you found it. Now let me ask you a question "how many time do you report mildew in showers to your customer"? This too is a mold type substance but it is something we are not surprised to find and probably don't even mention. Report on what you, as a home inspector, have been trained to report on and be careful in being specific about things you haven't been trained on. You know its mold. I know its mold. If you report it as mold then you open a whole can of worms of what type of mold is it? How do I get rid of it? Is it dangerous? The proper response is "I don't know I'm not a trained mold inspector".