Metal transition strip between bathtub/shower and wall

Question

I was trying to sand down my bathroom walls before painting. I have a fiberglass bathtub/shower that transitions to the wall with a piece of thin aluminum in between. There was an L-shaped piece of painted wood molding covering the strip-wall transition (pictured leaning against tub) which I removed in order to be able to get my sander to fit in the gap. But the thin layer of plaster on the strip is starting to crumble in places.

What am I dealing with here and how can I shore up this aluminum strip thingy?

Edit: it's not just a side strip, it appears to extend to the whole wall above the shower.

Answer 1

I didn't want to redo all the plaster so I gobbed some 3M Large Hole Repair spackle into the broken areas. Leveled it and sanded it down and it should be good after painting and after the wood trim is back on.