The Independent Colonial Strategic Alliance (ICSA) is an unaligned version of the Interplanetary Strategic Alliance, created to give completely independent worlds some sort of support.
The ICSA seems to make few demands and offers large amounts of help, at least to the Helghast. The ICSA offered support to the Helghast decades after the First Extrasolar War, but all offers were refused by the understandably paranoid Helghast.
During the Second Extrasolar War, the ICSA pleaded that there should be a diplomatic solution to the conflict, weeks before the ISA invasion of Helghan. These pleas seem to have been ignored, possibly a result of the ICSA alienating itself in the first place.
The ICSA is mentioned twice in some intel. Both mention "the breakaway colonies of the ICSA", which suggests the ICSA is not just some obscure organization and is actually fairly successful in its efforts to support colony worlds that attempt independence. The Phantom Talon Corp is said to have been involved in disputes on ICSA colony worlds.