It depends on the style of massage used, and how severe the excema is. Honestly, you can receive massage treatment as long as the excema isn't causing you pain when it is touched or when you move your body. However, you cannot receive any hot therapies because they will aggravate the excema. That means no hot towel treaments, paraffin dips, hot baths, most spa treatments, etc on your back, but you can on other areas of your body as long as the excema is not present in that location. Also, I wouldn't recommend any deep tissue tecniques since that could cause the skin to split and bleed, instead you should receive a relaxation massage because the oils and lotions can help hydrate the area and allows you to receive a massage without expediting your condition. Please make sure that your LMT is also aware that you have excema because you may need special products in order to help your body from breaking out in more rashes.
I also have excema as well as being an LMT, so I know this from firsthand experience. I hope that this helps!