Store the line handles returned from boxplot and plot and use them to explicitly tell legend which lines to use:
SOC_ = randn(10,1);
SOC_Max = randn(10,1);
SOC_mean = randn(10,1);
figure
h_bp = boxplot([SOC_, SOC_Max, SOC_mean], 'Labels',{'SOC','SOC(Max)','SOC(mean)'});
hold on
meanSOCs=mean([SOC_, SOC_Max, SOC_mean]);
h_m = plot(meanSOCs,'-o');
ylabel('State of charge (%)')
grid on
hold off
legend([h_bp(5,1) h_m],["Data", "mean"]) % using a blue box line h_bp(5,1)
