Yes; be concerned, but just keep an eye on it. Crazing is nothing more than minute cracks and acrylic is subject to crack propagation. Many things could cause such expansion of the minute fractures, e.g.; too much weight, an impact anywhere on the panel, the wrong chemical coming into contact with it, etc. Acrylic absorbs moisture up to a point. This means that one surface will have absorbed moisture (inside the tank) and the other surface (outside the tank) won't have moisture. The result is that slight bowing can take place on the moisture laden side. This puts stress on the tank and stress may result in crack propagation of the crazed section.
You can try re-enforcing that area with cement normally used when cementing two pieces of acrylic together, such as MDC. That trick can, sometimes, work to fill in and bind the micro fractures.
You may be ok, but I'd put a water sensor in that area just in case. A dramatic failure probably wouldn't happen immediately. It is more like to start cracking and leaking slowly ...in time for a sensor to alarm you.