What could the problem be? Thanks!
Planes with transparency on them will always look this way in the viewport.This is because, as a way to be able to render the view port quicker, blender simplifies the transparency: instead of rendering the gradient from solid color to transparent, it renders a gradient going from solid color to grey, and only the fully transparent parts of the image will show the face behind them. This is expected behavior for the viewport.
When you render the scene, it will render the transparency as expected.