I am learning a baroque piece on the recorder. Some notes have an unusual ornamentation symbol. How is a note with a plus sign over it played? It was suggested to play it as a mordent. Is that correct?
Not contradicting @DrSvanHay's source, which actually only arguments, that a trill may be marked as +: This is just indicating any kind of embellishment, I'm not sure, whether due to sloppiness of the composer or copyist, but I frequently find it also in fast movements on very short notes, where a trill will never fit in.
As example see here in a Boismortier print from 1737:
Note also, that in baroque much more was left to the performer and absence of such a sign does in no way inhibt it. The word mordent is also not really that stable in its meaning over the course of the centuries, but it may well be the appropriate one in your piece.
Summarized: either you simply choose an appropriate embellishment depending on the context(what other types of embellishments are present), or you may invest effort to find out, what experts say in that respect or how modern editors rendered the piece in different editions.
HENKE, Stephanie Morgan. A Study of the Early 18th-century French Baroque Musical Style: An Oboist's Performance Practice Guide to Jacques-Martin Hotteterre Le Romain's Troisième Suitte de Pièces À Deux Dessus, Pour Les Flûtes Traversières, Flûtes À Bec, Hautbois, Et Muzette, Op. 8. 2012. ph.d. thesis. University of Georgia.
says: (bold type by me)
The Tremblement The general French Baroque term for trill is tremblement. The tremblement is a rapid oscillation between the main note and its upper diatonic neighbor. It was most often indicated by a stenographic sign (+) which may appear above or just before the note to be trilled. Michel Pignolet de Montéclair wrote in his Principes de musique that the trill is "indicated in foreign countries and in music printed in France by a small t. Apparently, negligence in curving the base of the t resulted in the small + or x
where the "t" story is a quote from
ANTHONY, James R. French baroque music from Beaujoyeulx to Rameau. Hal Leonard Corporation, 1997.