It does look like Lymphocystis, but it can be difficult to tell. If it actually is Lymphocystis, there is no effective treatment, other than to keep the fish as stress free as possible. However, it is also possible that this is a fungal or bacteria infection, and these can be treated. It's worth a try, and I would recommend treating in a quarantine tank.
The belly getting larger may or may not be a problem, since it's also possible your dish is a female and filled with eggs.
As a note, you may get better results posting this on a forum that deals more with FW angelfish and cichlids. RS is mostly a SW reef forum, and while a lot of us also keep FW tanks, and will try and help, this is not an area of expertise for most here.