Mermaids

Mermaids are what Ilsa calls a Drowned. (Or a Water Zombie)

playing minecraft :) (episode 1)
A "mermaid" hits Ilsa while she is underwater.

the comments teach me minecraft :) (episode 2)
Ilsa is told by a comment that she should put doors on her waterfall door so a drowned doesn't get in. This is when Ilsa states she likes to call them mermaids.