John,
Do you have any screenshots you could provide? You also mentioned a masked video and window. As far as not being apply a mask to the group, there must be a bug. Also, not sure the mask tools are available for use on text. Could be wrong. If the mask tools are not available you can apply an image mask to the group.
Is the video behind the window? Sorry for the questions, just trying to understand what is going on here.
Here is another suggestion that may or may not work, try using the rectangle shape tool to draw shapes around the window where you want the moon to be visible. This may require multiple shapes. Place those shapes in another group. Apply an image mask to the moon, and use the shape layers as the mask. Let me know if this helps.