Although Comoros are a rather liberal Muslim country, in Comoros culture, it is disrespectful for women to expose their shoulders, much of their chest, knees, and especially stomach and lower back. Wear shirts or shawls that cover these areas. Locals will not expect foreign, non-Muslim women to cover their heads. When swimming, local women are fully dressed.
Foreigners are not expected to do this, but shorts and a swimming shirt is more respectful than a bikini or topless swimming. Men should wear shorts below the knee, though this is less offensive than a woman doing so. Public affection between men and women is not acceptable, though one may rarely see a Comorian man and woman holding hands briefly (in the nightclubs some locals seem to ignore Muslim convention).
Non-Muslim religious proselytizing is illegal, as is giving Bibles to locals. Locals are very tolerant and friendly towards non-Muslims, but avoid appearing as if you are trying to convert them.
Drinking alcohol in public is disrespectful, though it occurs in nightclubs. Restaurants generally do not serve alcohol unless they cater to foreigners.
To greet an elder, you say "kwesi". The elder says something like "mbona, mkana baraka" to which you respond "salaama".
It is a big mistake to hand out sweets to children on the street. Since locals are unused to tourists, this rarely occurs and they are usually just happy to talk with you, children included. Once tourists begin handing out gifts and money, locals will see Westerners as rich and free with money, destroying many opportunities for a human connection with them.
Children will harass tourists for candy and money (they occasionally do now). Tourists who do this are showing themselves to be disrespectful of locals (by assuming that money/sweets are what they want from tourists and by putting that in between them rather than making an effort to get to know locals) and ignorant of the consequences of their actions.
Since, allegedly, it was discovered that a Western man, resident of Grand Comore for 14 years, had been making pornographic videos and photographs, as well as violating children on the islands, the residents are quite averse to being filmed or photographed.
Individual reactions may vary upon being photographed, but visitors must be advised that taking unauthorised photographs of the locals will, at best, offend an individual and, at worst, cause irrational and potentially violent reactions in the subject.