Cruiser bottom bracket is clicking and grinding
Yeah, the first thought would be a loose crank arm, and if it isn't tended to immediately (if it's not too late already) the crank arm and possibly the spindle will be destroyed.

The other possibility, if this is a cartridge BB, is that the cartridge is slightly loose.  But in that case the noise is usually only present under high load.

[I'll add that tightening the crank arms is one of the half-dozen or so items that should be taken care of during the free once-over (after a few months use) that you should get from a bike shop if you buy a bike there.]