You don't want the moss to be acidic so the pH shouldn't be changed noticeably by addition of the moss. A pH under 6 would be quite acidic and something to be avoided.
To best test the pH of the moss you should allow a bucket of distilled/RO water sit out overnight so that it will equilibrate with atmospheric CO2 which forms an acid with water and its formation and dissipation can change pH. Take the pH of this water and then add the moss and let it soak for 24 hours. After this time period test the pH of the water again. There should be no appreciable change in pH. Large drops in pH would indicate acidic compounds in the moss and increases would indicate alkaline compounds
Using distilled, RO or other very soft water is important as there are compounds in hard water that act as buffers. This makes it difficult for the acid from the moss to change the pH and acidic moss may not leach out enough acids at once to change it, especially when the water has significant buffering capabilities.
