Mass Effect 3 Silean Nebula: Rings of Alune side-mission can be completed immediately after Priority: Thessia.
For this specific side-quest, you must go to Huerta Memorial Hospital, where you have to find an Asari consultant, in the Patient Lounge, near the entrance, to the right.
Get near the consultant to start Silean Nebula: Rings of Alune mission.
When to start
After Priority: Thessia
“An Asari consultant on the Citadel needs the Rings of Alune.
Find them in the Silean Nebula and return them to her at the Huerta Memorial Hospital.”
To find the Rings of Alune, go to the CIC on Normandy and open your Galaxy Map.
SW of Serpent Nebula you will find Silean Nebula.
Travel to that location, and then to Teyolia. Scan Planet Nevos and send a probe to retrieve the rings.
If you need additional credits (10.000) scan the large Planet Quirezia in the same system (Teyolia).
Next, return to the Citadel and visit Huerta Memorial Hospital.
Find the Asari consultant and give her the Rings of Alune.