I have been growing two avocado plants indoors for the past two years and recently, they have developed a orange/rust looking film on the side of the pot. Can anyone help me and tell me why this is happening? They are in glass containers (and have out grown them I have been meaning to replant them for a while now but want to eradicate this film before hand?) and sit in the light for the majority of the day. Overwatering is most likely not the issue, but I am open to any suggestions. Attached is an image of the film around the rim of the pot.