mouse.x refers to an undefined (user defined type?) variable. If "mouse.x" was a symbol that was associated with a value of 5, you would need to assign it to a string variable, then you could search the symbol with instring() to find its components. From this you could work out the value 5.
a+c concatenates the strings just like the operator a&c does.
Seriously, why might you want to do this? Would it not be easier just to