It could be a bite or even a burn, slight chance that it's fungal but I doubt it's Y.F. More likely a bite since they are usually housed with siblings at the stores.
Yes those are no good -- if you want to still use a mvb your better off going w/ a power sun or a mega ray but you will still need to get another long tube fixture uvb - mvb's do not emit enough uvb - it's best to have a long tube fixture as well to supply adequate uvb