Circuit boards are delicate. They need to be protected from water and kept away from cleaning agents. They also need to be protected from static electricity when exposed. Take the same precautions you normally would when removing or exposing the board (such as wearing an anti-static wrist band). Then gently vacuum up loose roach droppings. Dip a small, soft nylon brush in isopropyl alcohol, remove excess alcohol, then carefully rub the material that remains.