Is it good or bad if they in tank with fish and live rocks only?

I would rid the tank of it

if you get lots, then use a Harlequin shrimp

Oops I've been calling these Astrea starfish for a while... I have had them in my tank for about 4 years. So far they have been model citizens and are no more prolific than Stomatella snails. At any given time I could probably spot 10 of them on the rocks and glass.

Here is what Calfo and Fenner had to say:

"Asternia folium and like species: This tiny asymmetrical sea star perhaps does not belong in either the best or worst categories of sea stars, and is surely misunderstood. Nevertheless, it finds its way here into the less favourable category if for no other reason than for its inclination to be excessively prolific. Asternia has been accused of grazing some coral, which surely occurs in some cases. More often than not, though, they graze necrotic tissue (mistaken for killing an already dying coral) and algae, and even large populations have been maintained in aquaria with absolutely no harm to corals- effectively reef-safe. However, even when well behaved, the very likelihood of rapid proliferation makes this sea star a nuisance of possibly plague proportions, and amnesty from grazing live corals ultimately cannot be assured. It is unfortunate because they are by and large dedicated algae grazers and quite useful otherwise in aquariums. They may even be cultured as suitable prey for the Clown Harlequin Shrimp, Hymenocera sp. In refugiums and fish-only displays, this sea star can be a fascinating and effective scavenger. Monitor the population of this starfish in all aquariums."

Reef Invertebrates 2005 p361

This has been my experience. As I said, I have hundreds of them, no issues at all. They will eat algae off the base of any receding coral, but do not eat healthy tissue. Mostly found grazing on algae on the glass when I forget to wipe it off for too many days. Found mostly on the rocks, I find they do a great job at keeping my rocks clean. For me, I would intentionally introduce them to a tank.

Here I go. If any Edmontonian have these starfish and want to get rid off.
please give some to me.

Thanks,
Kev.

they dont grow bigger then that
and they seems safe reef

Congrats!! You got a free star which will stay the same size.
Interesting to watch, reef safe. Good!

I only have one. Will it multiply?

'
Yes!

Good! Sit back, relax, and let it multiply. 2x2, 4x4, 16x16...Then scream after.