It is too obvious, but that is what it is. Keep in mind that when you are constructing things you are adding elements as you need.
In that case, you have a photo, then you add a transparent rectangle, and IF you need it, you add text on the top of it, or you add a logo, or you add something else. The transparent rectangle is independent of the text.
Of course, you are adding it to increase the contrast between the text and the background, but your objective the reason you used it, IMHO, does not change the element itself.
But if we take an editorial design point of view, that element is probably the background of the container of the text.
I some programs like Indesign it could be a "transparent background of your frame", and if you are doing some web design using CSS it could be a "transparent color for the bounding box". In both cases, the containers can normally hold text.
Here are some links. Not really to show how to do anything, but about the usage of the terms "frames" and "box" in different situations.
In some other programs or situations (like in a real-life interior design of a window for example), you would just add... a "transparent rectangle" (or any other shape for that matter).
I don't think adding the term "card" or additional names help in any way. It gives too much context to the definition making a definition too narrow.
In your question, you use the word plate... A plate is too specific and does not really describe the transparent nature of the rectangle. A "plate" is a way of calling what in other programs could be "layer" or "object".