That's normal for a Plus. The video circuits spew out whatever's in DRAM until it gets into the boot sequence a bit. The pattern can change, depending on what was last displayed, and how recently it was displayed. As long as it proceeds to asking for a boot disk, and then boots properly, I wouldn't worry about it.

And if you want to speed up booting, you can bypass the memory test by holding the mouse button down on power up. Release after it starts the normal boot sequence.