In regards of what you said about tetris and how people make spin-offs. This is exactly the same as your case. People took the exact idea of tetris, changed graphics and thats it. It still has the increasing difficulty as the player progresses throught the game...where the blocks start falling faster. It still has the same blocks where you have to turn them. It still has the same idea about how you have to fill up a row...etc

This resembles your case. They took your idea and made a spin-off off of it. I don't see the harm done. It's a simple flash game.

Also note that you would end up spending more money with the lawyer.

Good luck.