Oh, so things are getting a little clearer. Chitose was born a normal human and turned into a mermaid, just like Haruna (except Haruna couldn't go all the way). That means the Houko aren't Chitose's fault. Some people in the town simply have a propensity to turn into mermaids, but some get stalled halfway. Grandma Reiko was okay with Chitose hanging out with Ichika because she too was Chitose's friend when she was young. She knows the mermaid isn't as dangerous as people think.
I think Ichika's sudden turn in chapter 1 is also not Chitose's fault entirely. It isn't the mermaid that's brainwashing people - it's drinking the lake's water. The lake water is probably also what's turning girls into mermaids.