I'd call the police to make a record, dumping stuff is a trespass and worse. I'd call the company president after making the police report and ask that they remove the grout and offer some sort of compensation of the plants suffer.

That is probably very alkaline. I would remove everything I could and then hose down really well.

And send a bill to the property owner next door for the ruined plants

Was this a recent thing of some time back. If recent then I agree with Lucille, call the company or have the neighbor call the company and make them dig it out.

If this happened some time ago then you are likely too late to claim damages or repairs and will have to dig it out on your own.

You don't indicate where you live or even you garden zone so it is impossible to know if you already had plants planted there or what.

Either way it is your property so someone other than you has trespassed to do it.

Dave

You could serve your neighbor a "Knuckle Sandwich" instead of a "BLT"!!!, ! also serve it to chew that dumped the stuff in your mater bed!

Gary