Change faces when chatting?

I need help figuring this out. What I want to achieve is, if somebody chats anything, their face changes its decal until it is finished. I haven't found anything on it, and I can't figure anything out.

There's not a specific event that fires when your chat bubble vanishes, but we can add a wait

local GreetingSynonyms = {"hi", "hey", "hello", "howdy", "whats up?", "what's up?"}

local Id = 1230123 -- your id

game.Players.PlayerAdded:Connect(function(plr)
	plr.Chatted:Connect(function(msg)
		
		local OriginalFaceTexture = plr.Character.Head.face.Texture
		
		for i, v in pairs(GreetingSynonyms) do
			if string.lower(msg) == v then
				print(plr.Name .. " greeted option: " .. i)
				
				plr.Character.Head.face.Texture = "rbxassetid://" .. tostring(Id)
				wait(2)
				plr.Character.Head.face.Texture = OriginalFaceTexture
				break
			end
		end
	end)
end)

There is no event on the Player that detects whether the player is typing in the chat or not. But, you can also use the Userinputservice to detect the Enum.KeyCode.Slash. This is the keycode that opens the chat so you can actually take advantage of this and make the script you wanted.