My dad used to swear by well rotted horse manure around his roses and yes they will benefit from it. Do you mean the white fungus in the horse manure? Thats not a problem.

So long as its well composted, preferably with straw rather than wood chips, roses will like it very much.

Ignore the fungus.

Thankhu x

Horse manure is whats known as hot manure this is due to the horse's manure being more solid than cows,
cow manure tends to be sloppy and wet hence it goe's cold quicker,
Horse manure tends to keep warmer in the winter when used for the likes of roses hence its a good mulch plus a good feed.
Rose's love it.