Does an Iranian with German residency need a visa to enter Romania?

As stated by Timatic, the database used by Airlines:

Visa required, except for Passengers with a residence permit issued by Germany for a maximum stay of 90 days

So no, you do not Need a visa; at the border just present your passport and residence permit

No, you don't need to get a visa if you fall into one of the categories listed below:

Holders of Schengen visas with two or multiple entries, national visas or residence permits issued by Schengen Member States.

Starting July 11, 2014, in keeping with the provisions of Decision No 565/2014/EU, Romania acknowledges as being equivalent to its own national visas, for transit and for stays up to 90 days per period of 180 days the following:

• The two or multiple entry short-stay visa issued by a Schengen member state;

• The limited territorial validity visa issued by a Schengen member state;

• The short-stay visa issued by Bulgaria, Cyprus or Croatia;
• The long-stay visa issued by a Schengen member state;
• The long-stay visa issued by Bulgaria, Cyprus or Croatia;
• The residence permit issued by a Schengen member state;
• The residence permit issued by Bulgaria, Cyprus or Croatia
• The long-term resident's EC residence permit (the validity of which is of 5 years or more) issued by a member state of the European Union.

Source: Romanian Ministry of Foreign Affairs