I'm not sure what it's made from but to me it looks like plastic, but I could be wrong.

I just had a pool guy look at it today and he said I probably can't fix it with the liner in place and water in the pool. It makes sense since the liner sticks into the gap that the corners connect to. Oh well, I guess I'll have to wait till I replace the liner one day which will be hopefully many years from now as it's new.