Finger Coral Pests?

[aras]

My finger coral hasn't opened for 3 or 4 weeks. Some places now started to get covered in brown patches (algea?) so really not looking good. None of the other corals seem to be affected so I was wondering is there were some pest involved...
Then, finally this evening I noticed some small creatures on the coral. Don't think these are flatworms (different shape, move too fast)... Any ideas what they are and whether they could be damaging the coral? Here is a short video.