Here are a couple of pics. Thanks in avance for the help!
Attachment 18335Attachment 18336
Now, if we had a picture of the part in the wall behind the trim plate, we might be able to answer the question.
The cartridge is of course Grohe. Some companies provide valves to aftermarket "foo-foo" designers who provide the trim; I don't know if Grohe does that. Someone may recognize that logo just above the word "warm" on the plate. In any event, you do have 3 chances to get new trim....slim, fat, and none. But since based on the "modern" finish, I presume it is not so old, so there may be some grohe stuff that would work. Be aware that Grohe, and Hansgrohe, are two completely different companies.