After trying the hole nigth I did It! The problem was that the original material (quicktime apple prores 422, 1920×1080) was not optimized for Fina Cut Pro ( a new this cause when I import the material to the FCP a long window appears saying so, but I didn´t give it importance cause let me import anyway) So, I import, export the specific shot from FCP with the same codec… then import that shot and then send it to Motion.
The mask export perfect.
Thank you