set(H,Name,Value) specifies a value for the property Name on the object identified by H.
The string(name) you are creating is of type char. You cannot use this value directly in set command instead, you can use this value to find the objects with the same Tag name and set their visibility to 'Off'
for i = 2:x
name = strcat('SoH_C',num2str(i),'_1')
Hope the above helps