Name a parasitic alga.

Explanation

**Step-by-Step Solution:**

1. **Identify the Type of Alga**: The question asks for a parasitic alga, which is a type of algae that derives nutrients from a host plant.

2. **Recall the Genus of Parasitic Algae**: One well-known genus of parasitic algae is *Cephalosorus*.

3. **Understand the Characteristics**: *Cephalosorus* is a genus of phthaloid algae that contains approximately 14 species. It is commonly referred to as "red rust."

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