Mealy bugs: These little critters are actually soft bodied scale insects that produce a powdery wax as they feed on the sap of your plants. In Epiphyllum they are most often found on new blooms and in the crevices of branches. Light infestations although unsightly, are usually harmless but left untreated can cause severe damage to your blooms.
Solution: Small infestations can be sprayed with a solution of 50% isopropyl alcohol and 50% water several times a week while insects are present. Large infestations can be controlled with an insecticidal soap spray or systemic insecticide (follow manufactures directions for application rates and times). Light Horticultural oil is also very affective against mealy bugs but it is becoming very hard to find for the home gardener (CAUTION: Neem oil is too heavy for Epiphyllums and may cause damage to your plants)
Biological control of mealy bugs can be obtained by releasing Ladybugs or lacewings at the bottom of your infested plants at dusk.