Light Grey + a little bit of purple flatworm....Good or bad??

[Piscean]

I just found this really cool flatworm on my LR.. I went in to check it since it's night-time and all the cool creatures come out.

It's grey-ish...with dark purple details.

I'm going to post a pic that I found on the internet...It's looks similar to that, but SLIGHTLY darker!

So, is it good or bad? I heard some are harmless...?

THANKS!

-James