How to fix this plaster issue?

[Kcdc42]

What is the best way to fix this plaster issue? I took this picture before closing the pool. It is around 4-5 inches. What’s the best way to fix this? Can this be a DIY job or does this require a professional?

I am draining my pool when opening and I was thinking of using the ‘Pool Patch - Plaster repair kit’ when empty before refilling the pool. Is that a good and durable product? Is this a feasible approach?

 

[AQUA~HOLICS]

It may get bigger when exposed to air, remove the unstable plaster until solid plaster is present.
This product is durable and commonly used for this type of application.