The group of words in bold "that hang from the ceiling of the cave" is a clause.
Explanation:
1. A clause is a group of words that contains a subject and a verb. In this case, "that" is the subject and "hang" is the verb, making it a clause.
2. Clauses can stand alone as complete sentences or be part of a larger sentence.
As for the complete subject in the sentence provided, it is "stalactites."