There are a few different Ancistrus that are known as white seams. The most common one is the L183 Ancistrus dolichopterus. They will fade in color during shipping and when stressed out, but will color right back up when settled in under the right conditions. Is the fish you received wild caught or captive bred? The reason I ask is because the 183 is a blackwater fish. If it is wild caught and it is not kept in blackwater conditions, then it may be stressed out and displaying the odd coloration that you are seeing. Most captive bred ones adjust to captive conditions without an issue. It could also be that it was misidentified.
