Knowledge of Edward's transformation is not foreign to him, the attack on the Dark Elf Kingdom back then made it very clear that Edward had turned to the dark side. He has been turned into something inhuman by the power of the Executor.
A sudden visit from him is an unexpected scenario.
However, Rex decided to use this opportunity to size Edward's new power.
Being turned by the Executor means that his power is of the same source, connected with one another. Thus, Rex assumed that if he knew more about Edward's power, he might as well gain crucial insight into the Executor's power that seemed bottomless.
Now, he couldn't find that insight, but it's definitely a start.
"What did you do to Gistella?" Rex asked gratingly, a hint of threat inside his voice.
Edward grinned in return, "I was only testing if you are connected to her"