Answer:
When Billy saw the seaman, he gasped and was visibly shocked to see the man in front of him.
The captain called the seaman "Black Dog".
Step-by-step explanation:
Robert Louis Stevenson's "Treasure Island" revolves around the character of Jim Hawkins and his adventures sailing around, looking for treasure. This treasure was buried by an evil pirate, Captain Flint, and focuses on the greed of humans in getting that treasure for themselves, even at the cost of other people's lives.
Billy is the 'captain' who had arrived at Jim Hawkins' family inn and has been staying there secluded in his own world and not interacting much with others. So, when the seaman called out to him, he seemed shocked to see that man in front of him. The narrator describes:
The captain spun round on his heel and fronted us; all the brown had gone out of his face, and even his nose was blue; he had the look of a man who sees a ghost, or the evil one, or something worse, if anything can be; and upon my word, I felt sorry to see him all in a moment turn so old and sick.
This change in his appearance was due to the seaman whom Bill called "Black Dog", who had been an ex-partner of the captain and was there looking for the map to the hidden treasure.