I presume your instructor is using the rule: "hyphenate an open-form compound noun when it is used as an adjective preceding the noun it modifies." Not everybody agrees with this rule, but you can find it online.

From the above link:

Use a hyphen to join two or more words serving as a single adjective before a noun:

• a one-way street
• chocolate-covered peanuts
• well-known author
  However, when compound modifiers come after a noun, they are not hyphenated:
• The peanuts were chocolate covered.
• The author was well known.

I presume your instructor is using the rule: "hyphenate an open-form compound noun when it is used as an adjective preceding the noun it modifies." Not everybody agrees with this rule, but you can find it online.

From the above link:

Use a hyphen to join two or more words serving as a single adjective before a noun:

• a one-way street
• chocolate-covered peanuts
• well-known author
  However, when compound modifiers come after a noun, they are not hyphenated:
• The peanuts were chocolate covered.
• The author was well known.

If you are writing for a publication where this is in the style guide, or taking a course where the instructor believes in this rule, you should probably follow it.