Tom himself refuses to be defined, but exists outside of time, predating even the elves. The fact that the Ring has no effect on him is remarkable. Even the Istari have something to fear from the Ring's influence. Perhaps the lack of effect is related to his position as a Master with great power, but without any ownership.
"He is, as you have seen him," she said in answer to his look. "He is Master of wood, water, and hill."However, for all his power, the sanctuary of his home is not absolute. The hobbits' dreams are touched by something outside themselves.
"Then all this strange land belongs to him?"
"No, indeed!" she answered, and her smile faded. "That would indeed be a burden."
Tom may be largely untouched by the darkness of the world at large... but I don't think Goldberry is. She understands fear and danger. Tom knows about the darkness, but it doesn't really touch him. Again, I also think that Merry and Pippin have a better grasp on what they're facing than Frodo does. I don't think it's just remembered fear of their encounter that makes them cry out to not speak of Old Man Willow at night.