It's hard to tell, however the very likely solution of your problem lies in a combination of two or three effects:
The image is not focused. Did you focus on bright stars or the moon and not change the phone afterwards? This a common problem and (I personally) find it hard with a phone to get a sharp image through a telescope. On the telescop I have available, I need to hold my phone about 1cm away from the eye piece to get a sharp image. The focus WILL need to be different than for you looking through the telescope with your naked eye. An un-focused image will be just that: bright area where like arrives and black outside, showing vignetting.
the image is motion-blurred.
you really took too long an exposure with too high an ISO rating. Reduce either and see whether you get better results. At highest ISO rating my DSLR camera yields bright white images after a comparable time (I didn't try my phone but suspect similar sensitivity behaviour). How long exactly depends on how dark your local sky is.
From all these points the most important is: make sure you have your camera-telescope-combination focused. Test that on bright objects, thus a sharp moon or smallest-possible dot for a bright(!) star imaged. You can do the focusing in video mode and highest ISO setting. When that is ensured, then only start to image interesting objects.