For falling seedlings and young plants I cut a piece of straw, open it up and introduce the stem of the plant inside, so you have a piece of plastic straw holding the weight, I will then either introduce it inside the rockwool (you should probably be able to put more soil to cover most part of it). After a few weeks, when the stem has become more solid I will carefully remove the straw.
Notice that you have to fix the problem too, get the lights closer if they are stretching. If you need a picture I will take one to my Blueberry which is about to get the straw removed and had the same problem during the seedling phase.