I would call it a custard, I think, if not only for want of a better word. A custard consists of milk, egg and a thickening agent i.e. flour, so it fits the description well enough.
It might sound a bit odd to English-speaking ears though. I think 'savoury custard' is probably more appropriate. I must admit I always thought moussaka et al was topped with a white/bechamel sauce; I'm pretty sure that's the more common topping in 'the West'.
As for the dishes themselves, I'm not sure if there is a catch-all English term for them - 'pasta bake' is an English version of the Italian pasta al forno, but that obviously only applies to dishes that use pasta.
Best Answer
Ốc bông = tiger moon snail (hotdeal.vn, Google images) or in Latin: Notocochlis tigrina (Wikipedia)
Ốc tỏi = Japanese bonnet snail (Facebook, Wild Fact Sheets, images, more images) or in Latin: Semicassis bisulcata (Wikipedia). Ốc tỏi is sometimes refered to as "garlic snail" (Hong Hai, Cooky), but this is obviously different from garlic glass snail, which is a land snail (Wikipedia).
Here on Facebook are Vietnamese/English translations for more than 100 types of seafood, including Ốc bông and Ốc tỏi.