usually clams spawning in the home aquarium is a survival instinct and a sign of stress. there are cases where normal healthy mature clams will spawn for other reasons but the outcome can be the same. all those gametes floating around the tank will eventually die and pollute the water as well as deplete all the O2.
do a water change and keep your skimmer running 100%, running carbon is also a very good idea to help with water quality.
any changes recently, accidental or otherwise???
how big is the clam?? and how big is your tank??