There are prophets who are seers such as "And Samuel answered Saul, and said, I am the seer: go up ..." I Sam 9:19. Being a seer just means that Yahweh reveals things to these believers primarily visually.
This shows the distinction -
"Now the acts of David the king, first and last, behold, they are written in the book of Samuel the seer, and in the book of Nathan the prophet, and in the book of Gad the seer,..." I Chron 29:29