Yes both of these sentences are correct.
However, the first sentence implies that you are going to see your grandma next / the coming Tuesday morning. If you’re wanting to tell someone what you’re doing this week, that would be the correct sentence to use.
If you use the second sentence, it implies that you visit her regularly on Tuesday mornings. For example, if she is in a care home and visiting hours are on Tuesday mornings. It sounds more like part of a routine.
Hope that makes sense!