On Erev Pesah, Halacha forbids partaking of Masa with which one may fulfill his obligation of Masa at the Seder. (This is to the exclusion of egg Masa, which may not be used for the Misva at the Seder, and which one may therefore eat on Erev Pesah.)
This prohibition results in an interesting Halacha in a situation when Erev Pesah falls on Shabbat. Since Masa is not suitable for consumption on this day, it serves no functional purpose and is therefore deemed "Mukse," something that one may not handle on Shabbat. Several authorities, including the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), Rav Haim Palachi (Turkey, 1788-1869) and Hacham Ovadia Yosef, have indeed ruled that it is forbidden to handle Masa on Erev Pesah when it occurs on Shabbat.
It should be noted, however, that this prohibition applies only to "Shemura Masa" Masa made from grain that had been carefully protected from water from the time it was harvested. Shemura Masa, which is what we use at the Seder, is particularly costly, and one therefore generally reserves it specifically for the Seder. Other Masa, however, which is less expensive, is often used on Erev Pesah to feed young children. As such, it serves a purpose even on Erev Pesah and may therefore be handled even when Erev Pesah falls on Shabbat.
If one must move Shemura Masa on Erev Pesah that falls on Shabbat, he should place an item that may be handled such as some other food product on top of the Masa. The Masa then becomes a "Basis Le'davar Ha'mutar" a "base" for a permissible item which Halacha allows one to move on Shabbat.
Summary: When Erev Pesah falls on Shabbat, it is forbidden to handle Shemura Masa on that Shabbat. Other Masa may be handled. If one must move Shemura Masa on such a Shabbat, he should place a permissible item on the Masa and then move the Masa.
