Is it on the sand?
There is a lot of missinformation about cucumbers.
Sea apples are basically one of THE MOST TOXIC animals we can keep. They are GREAT for species tanks...but not so great in just general tanks. YOu will have to dose liquid foods for this in order to keep it healthy. But if it is stressed, it has the potential to kill all of your fish.
Other cucumbers (and YES sea apples are cucumbers) are FAR less toxic than sea apples. Most are not much to worry about actually. They are either filter feeders or sand sifters. Sand sifters sift sand and eat organic matter and bacteria. Filter feeders like sea apples or other common cucumbers in the hobby need to have liquid diets (or small particles - rotifers, phytoplankton, etc).