Lexapro belongs to a group of drugs called SSRI's which affect the chemical balance in the brain to inhibit the reuptake of serotonin. Weed shouldn't bother the Lexapro at all since weed mainly affects the neuron receptors of the brain and Lexapro deals with the chemical balance. You shouldn't notice anything different except the usual affects of Lexapro.

When you come off Lexapro if your on it for a long time it can be like trying to quit smoking you get really moody and pissed off at everything. If anything the weed should help with your problems other than compound them.

P-Drugs 101 (CPhT)