Performant way to write lambda in std accumulate when init is string.

I was implementing some code that required something like this

const auto concat = std::accumulate(ints.begin(), ints.end(), string{}, 
[](string& acc, const int& val) { return string(std::move(acc))+to_string(val);});

2 questions:
1) is it safe to move from acc?
2) is it faster(than having const string& acc argument?