Mastic replacement - How important is it to remove ALL old mastic?

[Undeadpickle]

Hello, I am currently replacing all of our old pool mastic. I got the majority of the old stuff out but there is still a bit stuck to the side of the flagstone decking which is quite difficult to remove (see photo). Is it ok to leave or should I use a grinder or some other tool to try and remove it all completely? Thank you!

 

[kimkats]

I would leave it. I worry the grinder would do more harm than good with that little bit left.