Hello,
The Catalonian secessionists themselves don't clearly state why they want independence. The catchphrase "Spain is stealing from us" is one of their strongest points, but they avoid debating this in public because it's full of fallacies. Otherwise, it's one of the most prosperous regions of Spain, very industrialized and the Catalonians are definately an innovative and intelligent people. But they are not ethnically different nor significantly culturally distinct from other Iberian regions, and their economy depends largely on exports to Spain and the trade between Spain and France that goes through Catalonia. They are not more exploited, taxed or oppressed than any other Spaniard or EU citizen for that matter. In fact, it's probably one of the best parts of Western Europe to live in all things considered. The underlying motives for secession are more likely to do with creating a strong centralized state free from Spanish intervention, perhaps part of the region to become a tax-haven and to severely damage Spain's economy. Of course, this is just speculation on my part. Once independent we'll see how they treat those regions within Catalonia that will want their own regional independence or to reunite with Spain. This will certainly happen.
About immigration: Spain isn't much of a migrant destination as compared to France, Germany or the UK. Welfare for immigrants is limited in quantity and time as far as I can tell. Catalonia seemingly favours Muslim immigration over spanish speaking Latinos because Arab and Urdu speakers will prefer to learn Catalonian than Spanish and so fully integrate. I'm not sure this is true, but that's what I've heard and I've seen a ton of Pakistani, Senegalese and Morrocan immigrants in Catalonian towns and farms.
If anything, the Catalonians want Spain to take in more immigrants and refugees. Take for example this march in February 2017:
" More than 160,000 march in Barcelona to demand Spain takes in more refugees "
Nothing is as it seems, only critical thinking can save the West.