This could be translated as "If you had faith that was even as small as mustard seed" or "If only your faith was as big as a mustard seed, but it isn't." The construction of the phrase implies that they did not, in fact, have faith that was even the size of a mustard seed.
## be uprooted, and be planted in the sea ##
This could be translated as "uproot yourself and plant yourself in the sea" or "take your roots out of the ground, and put your roots down into the ocean."
## it would obey you ##
"the tree would obey you" This result is conditional. It would happen only if they had faith.