Single word for "enclosed in parenthesis"
Solution 1:
Refer to it as a parenthetical :)
parenthetical [ˌperənˈTHedək(ə)l] adj - relating to or inserted as a parenthesis
So, for your example:
X(1)
The parenthetical value(/number/figure) signifies the first slice of the tensor X
Although in this case, "the superscript" would also describe the same thing.