hi i had this problem about a month ago i simply replaced half of the water in my tank and took out most of the plastic plants that wear coverd in a dark green algae.
all was fine until yesterday when i put the plants into boiling water but did not scrub them.
algae remained.
i returned them to my tank and a day later the oil has returned.
so now i am off to just remove the plants again and then skim the top just for confurmation that the algae is to blame.
i will update in a day or two with my results