A noun modifier describes what a noun is doing, similar to an adjective; the difference is that an adjective does this in one word, while a noun modifier uses the second half the sentence to describe the noun in an irrefutable way.
A phrase that comes after the noun can modify the noun itself. This can be done very simply.
Take the following sentence for example: Please hand me my pick for playing the guitar.
By adding the phrase after the noun, it is clear what kind of pick is desired.
An adjective is a word that describes a noun. Any noun modifier needs to have an adjective in it, and the main word that modifies the noun can be removed, and placed in front of the noun as an adjective. Example: Please hand me my guitar pick. The noun modifier had showed that the type of pick wanted was for playing the guitar, the adjective makes it simpler by putting the descriptive word guitar in front of the noun.
Even though a noun modifier and an adjective perform the same fuction in a sentence, they are not the same thing. An adjective is a concise way of saying the same thing as a noun modifier. This doesn't make a noun modifier bad English, it just means that sentences can be structured in various ways.