das Mäuschen
little mouse
The diminutive form of Maus; a small mouse.
Ein kleines Mäuschen huschte schnell über den Küchenboden.
A little mouse quickly scurried across the kitchen floor.

little mouse / sweetheart
Loading your saved state.
little mouse
The diminutive form of Maus; a small mouse.
Ein kleines Mäuschen huschte schnell über den Küchenboden.
A little mouse quickly scurried across the kitchen floor.

'Mäuschen' (little mouse) is the diminutive form of 'die Maus' (the mouse). The ending '-chen' not only makes the noun small and cute, but also always changes the article to 'das'.
Hinter dem Schrank hat sich ein kleines Mäuschen versteckt.
A little mouse has hidden behind the cabinet.
You can recognize the meaning from the context. If people are talking about a small animal, the literal meaning is meant. If someone is addressing a person affectionately, it is a term of endearment. The fixed expression 'Mäuschen spielen' means always wanting to watch secretly.
Sei still, ich glaube, da ist ein Mäuschen in der Wand.
Be quiet, I think there is a little mouse in the wall.
As a term of endearment, 'Mäuschen' (sweetheart/little mouse) is used for loved ones, often for partners or children. It frequently appears together with a possessive pronoun such as 'mein' or 'meine'.
Keine Sorge, mein Mäuschen, wir schaffen das zusammen.
Don't worry, my sweetheart, we'll get through this together.
The idiom 'Mäuschen spielen' is a fixed unit. It expresses the wish to be present unnoticed in a private or secret situation. It is often used with 'möchte' (would like) or 'würde gerne' (would like to).
Wenn die Chefin mit dem Personalrat verhandelt, würde ich gerne Mäuschen spielen.
When the boss negotiates with the staff council, I would like to be a fly on the wall.
darling / treasure
'der Schatz' and 'das Mäuschen' can both be used as affectionate terms of endearment. 'Schatz' is very general and works for a partner or a child. 'Mäuschen' sounds cuter and softer and is used especially often for a child or someone close to you. It can be affectionate, but in some situations it can also sound too childish. As a simple guide: 'Schatz' is the more neutral term of endearment, while 'Mäuschen' is more strongly cute and diminutive.

Schlaf gut, mein Mäuschen, morgen gehen wir in den Park.
Sleep well, my sweetheart; tomorrow we're going to the park.
Danke, Schatz, dass du den Einkauf schon erledigt hast.
Thank you, darling, for already taking care of the shopping.