if they are not stacked on your mark, can they be destroyed?
Yes they can,
for example, you have a
mark in your deck
then you draw an unupped life mark card (which will stack with your markt sign) nothing happens, its indestroyable.
but if you draw an upped life mark card (which won't stack with your mark) you can destroy it,
and if you draw a (un)upped mark card which doesn't have the same element as your mark in your deck, you can destroy them.