Wouldn't worry too much personally, it's usually a good sign.
If it really bugs you, i'd just mulch the top and let it decompose.
It's a little hard to tell, but is that algae growing near the stock?
Might want to let the top dry out a little more, between waterings.
If you've been having to water frequently because the top soil's drying out too fast, again mulch will likely help.
Soil honestly looks healthy imo.