Hello Rodrigo,
Moving Average is an aggregated operator and it doesn't really work unless it is assigned to a visual like a crosstab or graph.
As dynamic text in a Text object, there is no aggregation context to work with. (Arguably, you should not be able to assign aggregated measures as dynamic text.)
In general, making a dynamic date selection based on the most recent date can be tricky. I seem to understand it is a limitation of how LASR and now CAS handle date values, but it is frustrating. Hopefully someone can help you find a good workaround.
Sam