Acetaldehyde is a naturally occurring compound produced by the oxidation of ethyl alcohol. During a vigorous fermentation it is generally not a problem and should disappear with a thorough punch down or other cap manipulation. If the aroma of acetaldehyde persists following a punch down it may be an indication of a lagging fermentation. Please monitor the progress of your fermentation carefully. Consider pressing if the ferment has slowed significantly and the Brix value is below 5°.
Acetaldehyde is an aroma you will want to familiarize yourself with since it is also an indicator of oxidative damage or spoilage in wines that have completed fermentation. To become familiar with this smell, set aside a small sample of wine and let it sit for a couple of days. The smell that develops is acetaldehyde.