You set one or more NoData values for your input source images in the Set NoData window.
To open the Set NoData window
In the Mosaic Tool window, in the control pane, do one of the following:
To set NoData values for all source images, right-click Source images, and then click Set NoData.
To set NoData values for one or more images, under Source images, select the image, group, or range of images you want, right-click, and then click Set NoData.