Term for words that appear to mean something that they don't?

Is there a name for words that appear to mean something other than their actual meaning?

For example:

• A "paper boy" is not a boy made of paper.
• A "greenhouse" is not a house that is coloured green.

They are generally an example of exocentric compound.

However, in another common type of compound, the exocentric or (known as a bahuvrihi compound in the Sanskrit tradition), the semantic head is not explicitly expressed.

A redhead, for example, is not a kind of head, but is a person with red hair.

Similarly, a blockhead is also not a head, but a person with a head that is as hard and unreceptive as a block (i.e. stupid).

And, outside of veterinary surgery, a lionheart is not a type of heart, but a person with a heart like a lion (in its bravery, courage, fearlessness, etc.).

More details and types of exocentric compounds:

http://www.victoria.ac.nz/lals/about/staff/publications/Bauer-EnglishExocentricCompounds.pdf

And if we want to be creative, we can call them "pseudo double-entendre". A phrase pretending to be a double entendre. And perhaps it becomes "double-pretendre".

As @CarlWitthoft points out there may be no single term for words used with alternative meanings, but there are terms that categorize how the alternate meaning came about. You may have many other examples in mind, but both the ones you gave are endocentric compounds in which one aspect of a thing serves to identify the whole. A specific term for this category of repurposed meaning is metronym.

"Paper boy" is an open compound of a metronym. The product newspaper is identified by an aspect -- that it's made of paper (as in "I read the paper today). The compound paper boy implies a boy who delivers the (news)paper by idiomatic convention.

"Greenhouse" is a closed compounded metronym, where the thing (greenhouse) is identified by one of its features -- that it contains green plants.

You may be looking for 'misnomer', but that word is used mostly for names, if I'm not mistaken.
Misnomer literally means 'wrong name'.

However, the examples you've given aren't really misnomers because a paper-boy does deliver paper, and I think it's fairly obvious to anyone that you mean 'paper-delivery boy' when you say paper-boy.
Also, a greenhouse (different from 'green house' - which would mean a green-coloured house) is named so because it contains plants.

An example of a misnomer is 'koala bear'. Koala bears are not bears and they aren't related to them either.

Other words which also mean 'misnomer' are: 'misname' and 'catachresis', but misnomer is more common than both.

ermanen covers the solid compounds, but there are two-word examples.

Non-semantically-predicative adjectives are adjectives that don't (strictly) modify the noun they're 'coupled' with (if there even is one). They may modify an understood noun, or even reference a state pertaining to the noun.

A quiet drink means that the surroundings are quiet.

A heavy smoker's smoking is heavy.

A proud occasion means that some proud people are attending.

A former president isn't necessarily the president of anything now. His presidency 'is former'.

A fake gun isn't a literal gun according to the primary sense. It's a member of the set of fake artefacts.

An invalid toilet is a toilet for invalid people.

A mere youth is a member of the powerless, cashless, hapless class of youths (Yeah, right).

The terms inherent and non-inherent are sometimes used, but there are better treatments.

This is covered in detail at Wordwizard.

There are other attributive uses of adjectives where the adjective takes a semantic role other than its usual one.

Evaluative adjectives such as good also have an interpretation relative to the noun they modify: As Aristotle pointed out, a good thief is not usually a good man.

(E E Coppock)

So we wouldn't say a good thief is good. This is the non-intersective property.