Answer:
Competition, because they compete to grow on the same space and both species would do better if the other were not present.
Step-by-step explanation:
Competition refers to interaction among organisms for same resource. Here, the competition could be for food, shelter or reproduction. Sponges and corals grow on the same type of substrate and hence, exhibit competition for "habitat". If present alone, both sponges and corals would exhibit better growth as compared to their growth in presence of each other.