Japt, 103 bytes
q Ä o_pÃøU¬v1
Creates an array of all square numbers up to the input and checksSeems to see if the array containswork fine for 2**54-2 in the inputJapt Interpreter but fails on TIO for some reason...
Japt, 10 bytes
q Ä o_pÃøU
Creates an array of all square numbers up to the input and checks to see if the array contains the input
Japt, 3 bytes
¬v1
Seems to work fine for 2**54-2 in the Japt Interpreter but fails on TIO for some reason...
Japt, 1310 bytes
1oUqq Ä,_p} øUo_pÃøU
Creates an array of all square numbers up to the input and checks to see if the array contains the input