Sounds like it might be a mold of some sort. I can't tell you any more without photos. Grab some and post in the plants forum maybe? The only issues I usually have with my emersed UG is BGA if there's too much standing water.
Tony, actually I don't use seiryu stone in my tanks with shrimp, as they do raise the pH and hardness of water. The rocks you saw were plain granite, which are perfectly fine for shrimp and don't look that bad. If you do use seiryu stone, make sure to monitor your pH, gH and kH, because it can mess your params up.