Sometimes it stays on one spot, and really isn't cause for concern. Other times, it will spread fairly rapidly, and might begin to over-take the rocks. Personally, after having fought a few nasty battles with this stuff, I'd get it out while it's in small areas only.
If you have some kalk
powder, make a thick paste with a little hot water, and put some paste on top of the algae. Cover this with an inverted clam shell (oyster, scallop, whatever - even a slice of pvc
) to deprive it of light. Leave it like this for about a week. This should kill the valonia.
Don't try to scrape it off the rock - if you burst the 'bubbles', there are spores inside which will be released.
Some people use Emerald crabs to keep it in check, personally, I don't think that's the best option. JMO