The valve is frozen in the closed position. It is green from corrosion. There is a hole under it open to the sea.
Congrats you have located a “through hull” and valve. You have discovered the “Good news!
Bad news!
” joy about boat ownership.
Bad news is “it is frozen”. Good news is it is closed. Bad news you got to un freeze it. Good news it can be done. Bad news in may need replacement. Real bad news, if you just leave it eventually the corrosion will win and the valve will open on its own. Then you have a, how did you state it, “hole open to the sea”
Yeah you get the picture. It goes on and on like that. It is the nature of compromise.
More good news. It is winter and possibly the boat is out of the water. This is the time to fix the problem. Multiple applications of PB Blaster or penetrating oil may help. Gentle urging of the valve or a little heat may work. If not then removal and replacement is the fall back. Before doing any demolition, I would definitely find out what the hose is connected to. It could be the
Holding tank. Cleaning that out first would be advised before disconnecting the hose from the valve.
Good luck and keep us informed. It really is ok. It is just one of those things we boat owners deal with so that we can enjoy sailing.