'ashita mori san koraremasu ka?' koraremasu can be 1) potential form of 'kimasu(come)' --- Can mori-san come tomorrow? 2) respect form of 'kimasu' --- Will mori-san come tomorrow? 'ashita mori san koremasu ka?' --- Can mori-san come tomorrow? Grammatically koraremasuka is said to be correct. But the problem is that the respect form and potential form become same. Then more and more Japanese are using potential form without -ra because the meaning is clearer. This is a controversial issue in Japan. But in the usual conversation, you don't need to worry about. Both forms are acceptable.

Korareru is the passive and potential form of come, so you could use it for things out of your control, i.e. another person or in the instance you might be able to come (somewhere). Koremasu is probably a mistake, I tried looking it up but didn't find a match, it could be kimasu instead, since kuru is one of those funny verbs that has all kinds of rules. Here is a link to the different forms of kuru in a big table, hopefully that will help. http://www.csse.monash.edu.au/~jwb/cgi-bin/wwwjdic.cgi?1W%CD%E8%A4%EB_vk

Both answers above are perfect. I want to add something (using the link above) it said: #) the ら is sometimes dropped from -られる, etc. in the potential form in conversational Japanese, but it is not regarded as grammatically correct. So こられます＝＝＞これます(Only in speech, and not grammatically correct). :)