I read that the sap in a desert rose is poison and not to get it into an open wound on yourself and to wash it off your skin asap.. My question is what kind of poison is it and the effects it could have on you? I can't find anything more about the sap/poison

This is a bit too complicated to answer here. Go to google and enter 'Apocynaceae poison' as the search topic - that covers a lot of the relatives of desert rose, which have some weird and wonderful compounds in them that act both as touch poisons and when ingested. (by the way, Apocynaceae is the plant family to which the desert rose, Adenium, belongs.) Derek