It wasn't a prior acquaintance, but rather Lucilia's distinctive hair color and ears. This appearance, clearly different from a normal human's, was the key to the Mermaid lowering her guard. In other words, this Mermaid was familiar with humans who possessed characteristics like Lucilia's, even though Alka had always thought Lucilia should be called an Elf.
Alka wasn't particularly surprised that Lucilia could speak the Merfolk language. After all, for a scholar like her, possessing some esoteric knowledge was quite normal. However, the fact that she revealed herself to communicate with the Mermaid clearly indicated her certainty that her appearance would make the Mermaid lower her guard. And indeed, that was the case.
