Looks like mold. Combine 1 gallon of water with 1 cup of white vinegar and scrub the deck, it should remove it.

thank you. I was so excited when I initially out the deck in and then had my heart broken when I saw this.

I'm looking to also have a Garapa deck installed since I couldn't find anyone in PA or NJ that sold Black Locust. Would be interested how it looked after you cleaned it, and did you seal it in any way after you cleaned the wood?

Agree - how did deck turn out and would you still recommend a garapa deck? Thx

Mold is typically not a common problem. We often see complaints of these small black spots during construction if someone was cutting or grinding any metal that would have been scattered around the deck. Metal dust will oxidize and leave tiny black spots on a hardwood deck that look just like this.