Whipped Cream Separates While Piping
Are you using an insulated piping bag with a disposable plastic piping bag inside?
Is the room hot?
Is the whipped cream over whipped?
To solve the heat problem, try folding a dissolved gelatin sheet into the whipped cream.
If the cream is over whipped it will start to look a bit grainy, as the fat starts to glob together as the fat turns to butter.