If it's just expelled dinoflagellates (zooxanthellae), which can appear as goopy, stringy brown glop trailing from the polyp mouths, then it's not that big a concern. Corals can and do go through the 'zoox pukes' now and then, particularly after an increase in lighting levels or temp. You can siphon away the goop, or not.
If on the other hand, you are talking about algal encroachment on tissue-denuded coral skeleta, then it is easier to simply break off the alga-infested skeleton, and let coral tissue grow over the break. The root problem in this case is WHY your Montipora gave up the skeletal real estate. Either from disease, injury/trauma or malnutrition, your coral was unable to hold ground, and THAT is serious cause for concern