I do enjoy mouse-precision type games like this, and I enjoyed myself enough to come back to this game after playing a bit last week.
If I may make a few suggestions, here's what I'd do differently. The fly sprite could use a single extra frame. Maybe just move the wings down two or three pixels and set the animation to be very fast, similar to how flies are animated in The Binding of Isaac. The movement could use a limit. Since you can move your mouse out of the bounds of the game and back in on another edge, you can pretty much teleport out of any danger. This can be fixed by moving the frog toward the mouse at a set rate rather than instantly. To alleviate the temptation to hide in the corners, you could perhaps make the map a pond with round edges. The flies can still travel the full screen, but the frog cannot. Lastly, I think the fly ingestion would be more satisfying if it ate all of them at once, and then flies would just spawn faster from there. Hope that helps :)