Geniescience
Homeowner
read the instructions on all the products you come in touch with before advising anyone over the internet. Another fiberglas tape product is made for cement board. It costs more. It may be in a different aisle. It won't fall apart in an alkaline environment.I've seen installers use different materials including nothing at all. I would not recommend using straight dry wall mud however as it could result in mildew growing behind your finished wall. Where ever you purchased your hardibacker board should be able to also provide the correct mud. Or could simply use stick on dry wall fabric tape and install the tile right over that.
2nd, whatever the "correct mud" is that you meant, could be named here in this thread. Telling people that they will get everything in the store including the right advice from an aisle person, is not sharp. Millions of bad installs have resulted from aisle people saying whatever they thought would work.
Welcome to this forum
david