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.
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.