Translation:Mr. Pöllövaara is looking for a small mouse.
Hiirtä is a partitive of hiiri, the same case as lasta or kahvia.
As you can see, Finnish nouns act slightly differently depending on the last few sounds and the age etc. of the word. Hiiri-> hiirtä goes like kuusi -> kuusta. Kahvi-> kahvia and lasi-> lasia are relatively new loan words so they don't change much, whereas vesi-> vettä and kivi-> kiveä are old words in the Finnish language.