it never says "instant spell" on cards.
This is incorrect. Look at Devoted Caretaker, Hidden Gibbons, and Ichneumon Druid.
I still say "When Vying Mermaid comes into play, target opponent may play a instant spell without payings it casting cost" will not allow you to counter it for free (if that is your intent). Think about countering another card with a comes into play effect. For example, Uktabi Orangutan. If you counter it, it doesn't come into play, and therefore it doesn't destroy an artifact. With Vying Mermaid, if you're able to play a blue instant for free, it has already come into play, therefore it cannot be countered for free.
Also, you forgot to change Vieing to Vying in the text box. And there are other spelling errors: "an instant", "paying its", "if it has X". (You didn't really change the text at all.)
And I still not sure about the name and creature type. I like Merfolk (and Mermaids), but WOTC has stopped printing Merfolk.