Depends on what texture the white edges have. If its smooth and similar in texture to the rest of coral, then yes it is most likely its growth edges. But if the edges are rough, what you are most likely seeing is its skeleton. A few things could cause that recession, either the monti is suffering from RTN(rapid tissue necrosis)/STN(slow tissue necrosis), it has been stung by another coral or that monti has monti eating nudis. Monti eating nudis normally feed on the coral starting from its edges since the nudis can easily retreat below the coral for cover.