Is white clouds dot the sky a possessive noun?

No, there is no possessive noun in the sentence. The nouns in the sentence are:

• clouds, a plural, common, concrete noun, subject of the sentence
• sky, a singular, common, concrete noun, object of the verb 'dot'.

A possessive noun is a form of a noun that shows that something in the sentence belongs to that noun. The possessive form is shown by use of an apostrophe s ('s) at the end of a noun or just an apostrophe (') at the end of a plural noun already ending with s. Example:

The sky's horizon was dotted with white clouds.

The clouds' whiteness dotted the blue sky.