Rub the smudge with a dry cotton cloth or dry paper towel. It will gradually burnish off as you present new cloth to the smudge.
Very few chemicals affect cured silicone including the normal soaps, detergents, and solvents.
Those that do dissolve it are too dangerous to be used around the aquarium. So mechanically wearing the smudge away is the safest method, even though it is tedious.