If you want to search for the phrase "coderanch mo*" in "caption", then the syntax would be caption:"coderanch mo*".
Sorry, I need to retract that. PrefixQuery (and thus the standard QueryParser parsing prefix queries) does not support prefixes with phrases, only with terms. So you need to use a query parser that does support this, namely
ComplexPhraseQueryParser which is part of the contrib/queryparser jar. For some reason that only worked with prefix phrases, not prefix terms, in my tests, so the solution needs to distinguish between those two cases:
If you add 3 documents (say, "coderanch", "moose" and "coderanch moose") you can see the difference.