They are different. When you are done talking to someone in person or on a Skype video call you can say 'nice seeing you' meaning "bye" but in a visual way. When you are talking to someone on a Skype call or when you meet them in person you can say 'nice to see you' because it is a greeting. Also, the next time you see them you can say 'nice to see you again'. Furthermore, you can add the word 'again' at the end of 'nice seeing you' given the same situation but after the first encounter with them. With the addition of a few words in the beginning or ending of either of these phrases can change the meaning, good question.

Hello! 'Nice seeing you' is in the past tense. This means that you had already saw them and as they are leaving you say 'nice seeing you' . 'Nice to see you' is in the present tense meaning that you currently just saw them and you are currently with them. 'Nice to see you' can also be a greeting when you first meet someone. Hope this is clear and helpful!!