Perce - I think what your describing is the lower spring cushion (or
isolator) - a foam rubber type of stuff that sits between the bottom of
the spring and the strut spring support cup - there's one on the top of
the spring too.  If that's the part, that's normal.  You could replace
it, but it will do that again before too long.  The part just doesn't
last long, but it's not critical.  I bet if you check your front ones,
you'll find them in similar or worse condition.  And for some reason
the lower ones fall apart a lot worse than the upper ones.

Bill Putney
(To reply by e-mail, replace the last letter of the alphabet in my
address with the letter 'x')