Lilli, yes, that's my understanding of what panspermia means too. If rocks from Mars do in fact occasionally fall on earth, it is quite conceivable that an animal of a water bear's calibre may hitch a ride across.
HOWEVER, all species on earth are linked through their genetic code to all other life on earth, and also to a common ancestor. And this ancestor would have been much simpler in design than a water bear. Therefore, earthly life could not have originated from water bear stock ex Mars.
Mintaka