Mota is used for 'grass'.
Regarding 'que mota' for 'really cool. . it is a hip expression certainly used in Madrid and probably the rest of Spain. Ben and Marina introduce a few 'cool' expressions like this -they say that if anyone uses them in other countries where they are not known, well, it is a basis for a conversation, hispanohablantes always like to hear what is going on in Spain (or wherever) and will be interested in hearing stuff like that.
ok I've found their 'statement' on this subject:
"Unless otherwise speci'ed, these phrases have come from the Spanish spoken in Spain. This means that if you rock up in Argentina, Mexico, or Peru, there's a small chance that one or two of them might cause looks of mild confusion when tried out on the locals.
Don't worry though, you won't get into any trouble, in fact the person you are talking to is likely to be so interested in hearing how a certain thing is said slightly differently in Spain, that they'll take the time to tell you their version in Columbian, Honduran or Chilean Spanish. When this happens, you end up learning even more!
o tell you their version in Columbian, Honduran or Chilean Spanis