What is the difference between modulo and modulus?
"modulo" is an operator.  For instance, we might say "19 and 64 are congruent modulo 5".

"modulus" is a noun.  It describes the 5 in "modulo 5".  We might say "the modulus is 5".

No, the two should not be used interchangeably.  It would be incorrect to say "19 and 64 are congruent modulus 5".  It would also be incorrect to "the modulo is 5".

See also https://en.wikipedia.org/wiki/Modular_arithmetic and https://en.wikipedia.org/wiki/Modulo_operation.  Both define the word "modulus", and as far as I can see they use it correctly.