Pull it deeper into the Eco complete? You don't have to worry about the crown of the plant being even with the top of the substrate, pull it an inch below. I haven't ever had loaches, my corydoras will move bits of substrate so an odd hairgrass plant goes floating but they aren't really digging. Could you make a loach playground somewhere in the tank? Put some pellets below the substrate so they dig there maybe? Could you put a strawberry basket with a rock on top to weigh it down over the new hairgrass?