As long as the sprites are overlapping and you use transparancy on them this so-called "glitch" will happen..
Transparancy reveals what is underneath (no glitch there)..
If what's underneath is not completely transparant, it will have a visible effect (two coloured windows behind eachother do the same..)
You could try changing the shape of your trail, try to prevent overlap or choose a different method alltogether..
But seeing you haven't got any gameplay in your game yet and the sprites/backgrounds etcetera aren't finished, you might want to leave this visual effect for what it is for the moment..