The bleach certainly did not cause the white porous condition.  It's a
question of whether the porous white area is the actual shell which has
become eroded by natural processes (in which case there is nothing you can
do to "remove" or correct it) or whether it is something that is attached to
the surface of the shell, "lime" deposits or colonies of some tiny colonial
calcarious organisms like bryozoans (in which case you can pick the white
material off with a sharp instrument and a certain amount of patience). This
may be easier while the material is wet.  Bleaching may also help loosen
such attachments - but you have already done that.

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